[RFC/PATCH] usb-storage: wait for device scanning before mounting root

[RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Pekka J Enberg]

From: Pekka Enberg <penberg@cs.helsinki.fi>

Add a new kernel config option CONFIG_LATE_ROOT_MOUNT that makes the kernel
wait until background scanning of USB mass storage devices is complete before
attempting to mount the root filesystem.

The config option is an alternative to the root_delay= kernel parameter
solution people currently use to boot from USB mass storage devices.

Cc: Daniel Drake <dsd@gentoo.org>
Cc: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Cc: <linux-usb@vger.kernel.org>
Tested-by: Vegard Nossum <vegardno@ifi.uio.no>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
---
 drivers/usb/storage/usb.c |    3 +++
 include/linux/init.h      |   12 ++++++++++++
 init/Kconfig              |    5 +++++
 init/do_mounts.c          |   32 ++++++++++++++++++++++++++++++++
 4 files changed, 52 insertions(+)

Index: linux-2.6/drivers/usb/storage/usb.c
===================================================================
--- linux-2.6.orig/drivers/usb/storage/usb.c
+++ linux-2.6/drivers/usb/storage/usb.c
@@ -928,6 +928,7 @@ static int usb_stor_scan_thread(void * _
 		/* Should we unbind if no devices were detected? */
 	}
 
+	complete_root_scan();
 	complete_and_exit(&us->scanning_done, 0);
 }
 
@@ -1007,12 +1008,14 @@ static int storage_probe(struct usb_inte
 		goto BadDevice;
 	}
 
+	begin_root_scan();
 	/* Start up the thread for delayed SCSI-device scanning */
 	th = kthread_create(usb_stor_scan_thread, us, "usb-stor-scan");
 	if (IS_ERR(th)) {
 		printk(KERN_WARNING USB_STORAGE 
 		       "Unable to start the device-scanning thread\n");
 		quiesce_and_remove_host(us);
+		complete_root_scan();
 		result = PTR_ERR(th);
 		goto BadDevice;
 	}
Index: linux-2.6/include/linux/init.h
===================================================================
--- linux-2.6.orig/include/linux/init.h
+++ linux-2.6/include/linux/init.h
@@ -147,6 +147,18 @@ extern unsigned int reset_devices;
 void setup_arch(char **);
 void prepare_namespace(void);
 
+#ifdef CONFIG_LATE_ROOT_MOUNT
+void begin_root_scan(void);
+void complete_root_scan(void);
+#else /* !CONFIG_LATE_ROOT_MOUNT */
+static inline void begin_root_scan(void)
+{
+}
+static inline void complete_root_scan(void)
+{
+}
+#endif /* CONFIG_LATE_ROOT_MOUNT */
+
 #endif
   
 #ifndef MODULE
Index: linux-2.6/init/Kconfig
===================================================================
--- linux-2.6.orig/init/Kconfig
+++ linux-2.6/init/Kconfig
@@ -473,6 +473,11 @@ config PID_NS
 	  Unless you want to work with an experimental feature
 	  say N here.
 
+config LATE_ROOT_MOUNT
+	bool "Wait for devices that do background scanning before mounting root"
+	help
+	  Say Y here if your root partition is, for example, on an USB disk.
+
 config BLK_DEV_INITRD
 	bool "Initial RAM filesystem and RAM disk (initramfs/initrd) support"
 	depends on BROKEN || !FRV
Index: linux-2.6/init/do_mounts.c
===================================================================
--- linux-2.6.orig/init/do_mounts.c
+++ linux-2.6/init/do_mounts.c
@@ -326,6 +326,36 @@ void __init mount_root(void)
 #endif
 }
 
+#ifdef CONFIG_LATE_ROOT_MOUNT
+/*
+ * Late root mounting for devices that need background scanning.
+ */
+static DECLARE_COMPLETION(root_scan_done);
+static atomic_t nr_root_scans;
+
+void begin_root_scan(void)
+{
+	atomic_inc(&nr_root_scans);
+}
+
+void complete_root_scan(void)
+{
+	if (atomic_dec_and_test(&nr_root_scans))
+		complete(&root_scan_done);
+}
+
+static void __init wait_for_root_scan(void)
+{
+	if (atomic_read(&nr_root_scans))
+		wait_for_completion(&root_scan_done);
+}
+#else /* !CONFIG_LATE_ROOT_MOUNT */
+
+static inline void wait_for_root_scan(void)
+{
+}
+#endif /* CONFIG_LATE_ROOT_MOUNT */
+
 /*
  * Prepare the namespace - decide what/where to mount, load ramdisks, etc.
  */
@@ -339,6 +369,8 @@ void __init prepare_namespace(void)
 		ssleep(root_delay);
 	}
 
+	wait_for_root_scan();
+
 	/* wait for the known devices to complete their probing */
 	while (driver_probe_done() != 0)
 		msleep(100);

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Alan Stern]

On Thu, 24 Apr 2008, Pekka J Enberg wrote:

> From: Pekka Enberg <penberg@cs.helsinki.fi>
> 
> Add a new kernel config option CONFIG_LATE_ROOT_MOUNT that makes the kernel
> wait until background scanning of USB mass storage devices is complete before
> attempting to mount the root filesystem.
> 
> The config option is an alternative to the root_delay= kernel parameter
> solution people currently use to boot from USB mass storage devices.

This doesn't take into account what happens when CONFIG_SCSI_SCAN_ASYNC
is set.

Alan Stern

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Pekka J Enberg]

On Thu, 24 Apr 2008, Pekka J Enberg wrote:
> > From: Pekka Enberg <penberg@cs.helsinki.fi>
> > 
> > Add a new kernel config option CONFIG_LATE_ROOT_MOUNT that makes the kernel
> > wait until background scanning of USB mass storage devices is complete before
> > attempting to mount the root filesystem.
> > 
> > The config option is an alternative to the root_delay= kernel parameter
> > solution people currently use to boot from USB mass storage devices.

On Thu, 24 Apr 2008, Alan Stern wrote:
> This doesn't take into account what happens when CONFIG_SCSI_SCAN_ASYNC
> is set.

Oh, right. Thanks! So something like the following should take care of it.

		Pekka

Subject: [RFC/PATCH] usb-storage: wait for device scanning before mounting root V2
From: Pekka Enberg <penberg@cs.helsinki.fi>

Add a new kernel config option CONFIG_LATE_ROOT_MOUNT that makes the kernel
wait until background scanning of USB mass storage devices is complete before
attempting to mount the root filesystem.

The config option is an alternative to the root_delay= kernel parameter
solution people currently use to boot from USB mass storage devices.

Cc: Daniel Drake <dsd@gentoo.org>
Cc: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Cc: <linux-usb@vger.kernel.org>
Cc: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
Cc: <linux-scsi@vger.kernel.org>
Tested-by: Vegard Nossum <vegardno@ifi.uio.no>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
---
 drivers/scsi/scsi_scan.c  |    2 ++
 drivers/usb/storage/usb.c |    3 +++
 include/linux/init.h      |   12 ++++++++++++
 init/Kconfig              |    5 +++++
 init/do_mounts.c          |   32 ++++++++++++++++++++++++++++++++
 5 files changed, 54 insertions(+)

Index: linux-2.6/drivers/usb/storage/usb.c
===================================================================
--- linux-2.6.orig/drivers/usb/storage/usb.c
+++ linux-2.6/drivers/usb/storage/usb.c
@@ -928,6 +928,7 @@ static int usb_stor_scan_thread(void * _
 		/* Should we unbind if no devices were detected? */
 	}
 
+	complete_root_scan();
 	complete_and_exit(&us->scanning_done, 0);
 }
 
@@ -1007,12 +1008,14 @@ static int storage_probe(struct usb_inte
 		goto BadDevice;
 	}
 
+	begin_root_scan();
 	/* Start up the thread for delayed SCSI-device scanning */
 	th = kthread_create(usb_stor_scan_thread, us, "usb-stor-scan");
 	if (IS_ERR(th)) {
 		printk(KERN_WARNING USB_STORAGE 
 		       "Unable to start the device-scanning thread\n");
 		quiesce_and_remove_host(us);
+		complete_root_scan();
 		result = PTR_ERR(th);
 		goto BadDevice;
 	}
Index: linux-2.6/include/linux/init.h
===================================================================
--- linux-2.6.orig/include/linux/init.h
+++ linux-2.6/include/linux/init.h
@@ -147,6 +147,18 @@ extern unsigned int reset_devices;
 void setup_arch(char **);
 void prepare_namespace(void);
 
+#ifdef CONFIG_LATE_ROOT_MOUNT
+void begin_root_scan(void);
+void complete_root_scan(void);
+#else /* !CONFIG_LATE_ROOT_MOUNT */
+static inline void begin_root_scan(void)
+{
+}
+static inline void complete_root_scan(void)
+{
+}
+#endif /* CONFIG_LATE_ROOT_MOUNT */
+
 #endif
   
 #ifndef MODULE
Index: linux-2.6/init/Kconfig
===================================================================
--- linux-2.6.orig/init/Kconfig
+++ linux-2.6/init/Kconfig
@@ -473,6 +473,11 @@ config PID_NS
 	  Unless you want to work with an experimental feature
 	  say N here.
 
+config LATE_ROOT_MOUNT
+	bool "Wait for devices that do background scanning before mounting root"
+	help
+	  Say Y here if your root partition is, for example, on an USB disk.
+
 config BLK_DEV_INITRD
 	bool "Initial RAM filesystem and RAM disk (initramfs/initrd) support"
 	depends on BROKEN || !FRV
Index: linux-2.6/init/do_mounts.c
===================================================================
--- linux-2.6.orig/init/do_mounts.c
+++ linux-2.6/init/do_mounts.c
@@ -326,6 +326,36 @@ void __init mount_root(void)
 #endif
 }
 
+#ifdef CONFIG_LATE_ROOT_MOUNT
+/*
+ * Late root mounting for devices that need background scanning.
+ */
+static DECLARE_COMPLETION(root_scan_done);
+static atomic_t nr_root_scans;
+
+void begin_root_scan(void)
+{
+	atomic_inc(&nr_root_scans);
+}
+
+void complete_root_scan(void)
+{
+	if (atomic_dec_and_test(&nr_root_scans))
+		complete(&root_scan_done);
+}
+
+static void __init wait_for_root_scan(void)
+{
+	if (atomic_read(&nr_root_scans))
+		wait_for_completion(&root_scan_done);
+}
+#else /* !CONFIG_LATE_ROOT_MOUNT */
+
+static inline void wait_for_root_scan(void)
+{
+}
+#endif /* CONFIG_LATE_ROOT_MOUNT */
+
 /*
  * Prepare the namespace - decide what/where to mount, load ramdisks, etc.
  */
@@ -339,6 +369,8 @@ void __init prepare_namespace(void)
 		ssleep(root_delay);
 	}
 
+	wait_for_root_scan();
+
 	/* wait for the known devices to complete their probing */
 	while (driver_probe_done() != 0)
 		msleep(100);
Index: linux-2.6/drivers/scsi/scsi_scan.c
===================================================================
--- linux-2.6.orig/drivers/scsi/scsi_scan.c
+++ linux-2.6/drivers/scsi/scsi_scan.c
@@ -1802,6 +1802,7 @@ static int do_scan_async(void *_data)
 	struct async_scan_data *data = _data;
 	do_scsi_scan_host(data->shost);
 	scsi_finish_async_scan(data);
+	complete_root_scan();
 	return 0;
 }
 
@@ -1823,6 +1824,7 @@ void scsi_scan_host(struct Scsi_Host *sh
 		return;
 	}
 
+	begin_root_scan();
 	p = kthread_run(do_scan_async, data, "scsi_scan_%d", shost->host_no);
 	if (unlikely(IS_ERR(p)))
 		do_scan_async(data);

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Greg KH]

On Fri, Apr 25, 2008 at 12:25:04AM +0300, Pekka J Enberg wrote:
> On Thu, 24 Apr 2008, Pekka J Enberg wrote:
> > > From: Pekka Enberg <penberg@cs.helsinki.fi>
> > > 
> > > Add a new kernel config option CONFIG_LATE_ROOT_MOUNT that makes the kernel
> > > wait until background scanning of USB mass storage devices is complete before
> > > attempting to mount the root filesystem.
> > > 
> > > The config option is an alternative to the root_delay= kernel parameter
> > > solution people currently use to boot from USB mass storage devices.
> 
> On Thu, 24 Apr 2008, Alan Stern wrote:
> > This doesn't take into account what happens when CONFIG_SCSI_SCAN_ASYNC
> > is set.
> 
> Oh, right. Thanks! So something like the following should take care of it.
> 
> 		Pekka
> 
> Subject: [RFC/PATCH] usb-storage: wait for device scanning before mounting root V2
> From: Pekka Enberg <penberg@cs.helsinki.fi>
> 
> Add a new kernel config option CONFIG_LATE_ROOT_MOUNT that makes the kernel
> wait until background scanning of USB mass storage devices is complete before
> attempting to mount the root filesystem.

What happens if there is no USB device present in the system?  Because
of that, there is no way that this option could ever be enabled by any
distro or anyone wanting their kernel to run on more than one machine :(

I don't like it.

thanks,

greg k-h

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Dharm]

[-- Attachment #1: Type: text/plain, Size: 2327 bytes --]

On Thu, Apr 24, 2008 at 03:56:04PM -0700, Greg KH wrote:
> On Fri, Apr 25, 2008 at 12:25:04AM +0300, Pekka J Enberg wrote:
> > On Thu, 24 Apr 2008, Pekka J Enberg wrote:
> > > > From: Pekka Enberg <penberg@cs.helsinki.fi>
> > > > 
> > > > Add a new kernel config option CONFIG_LATE_ROOT_MOUNT that makes the kernel
> > > > wait until background scanning of USB mass storage devices is complete before
> > > > attempting to mount the root filesystem.
> > > > 
> > > > The config option is an alternative to the root_delay= kernel parameter
> > > > solution people currently use to boot from USB mass storage devices.
> > 
> > On Thu, 24 Apr 2008, Alan Stern wrote:
> > > This doesn't take into account what happens when CONFIG_SCSI_SCAN_ASYNC
> > > is set.
> > 
> > Oh, right. Thanks! So something like the following should take care of it.
> > 
> > 		Pekka
> > 
> > Subject: [RFC/PATCH] usb-storage: wait for device scanning before mounting root V2
> > From: Pekka Enberg <penberg@cs.helsinki.fi>
> > 
> > Add a new kernel config option CONFIG_LATE_ROOT_MOUNT that makes the kernel
> > wait until background scanning of USB mass storage devices is complete before
> > attempting to mount the root filesystem.
> 
> What happens if there is no USB device present in the system?  Because
> of that, there is no way that this option could ever be enabled by any
> distro or anyone wanting their kernel to run on more than one machine :(
> 
> I don't like it.

This also has all sorts of races between do_mounts 'waiting' and the actual
USB device enumeration.  It's entirely possible that the kernel loads via
BIOS, the USB drivers are loaded, that forces devices to disconnect/reset,
and they take a while to re-enumerate.  During that delay, the kernel gets
to do_mount; now, no devices show in this "waiting for scan" count.

The right way to do this is via initrd and something like devlabel to wait
for a specific device to appear.  On some systems, you may want to wait for
several devices to appear.

I don't like this either.

Matt

-- 
Matthew Dharm                              Home: mdharm-usb@one-eyed-alien.net 
Maintainer, Linux USB Mass Storage Driver

What, are you one of those Microsoft-bashing Linux freaks?
					-- Customer to Greg
User Friendly, 2/10/1999

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Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Pekka J Enberg]

On Thu, 24 Apr 2008, Matthew Dharm wrote:
> This also has all sorts of races between do_mounts 'waiting' and the actual
> USB device enumeration.  It's entirely possible that the kernel loads via
> BIOS, the USB drivers are loaded, that forces devices to disconnect/reset,
> and they take a while to re-enumerate.  During that delay, the kernel gets
> to do_mount; now, no devices show in this "waiting for scan" count.

So how does that happen? ->storage_probe fails and driver core calls it 
later at some point?

On Thu, 24 Apr 2008, Matthew Dharm wrote:
> The right way to do this is via initrd and something like devlabel to wait
> for a specific device to appear.  On some systems, you may want to wait for
> several devices to appear.

You should be able to boot from an external USB disk without initrd. 
That's why we have the current root_delay= hack.

		Pekka

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Dharm]

[-- Attachment #1: Type: text/plain, Size: 902 bytes --]

On Fri, Apr 25, 2008 at 09:30:52AM +0300, Pekka J Enberg wrote:
> On Thu, 24 Apr 2008, Matthew Dharm wrote:
> > This also has all sorts of races between do_mounts 'waiting' and the actual
> > USB device enumeration.  It's entirely possible that the kernel loads via
> > BIOS, the USB drivers are loaded, that forces devices to disconnect/reset,
> > and they take a while to re-enumerate.  During that delay, the kernel gets
> > to do_mount; now, no devices show in this "waiting for scan" count.
> 
> So how does that happen? ->storage_probe fails and driver core calls it 
> later at some point?

There's no guarantee that storage_probe is going to get called in a timely
manner.

Matt

-- 
Matthew Dharm                              Home: mdharm-usb@one-eyed-alien.net 
Maintainer, Linux USB Mass Storage Driver

It was a new hope.
					-- Dust Puppy
User Friendly, 12/25/1998

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Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Pekka J Enberg]

Hi Matthew,

On Thu, 24 Apr 2008, Matthew Dharm wrote:
> > > This also has all sorts of races between do_mounts 'waiting' and the actual
> > > USB device enumeration.  It's entirely possible that the kernel loads via
> > > BIOS, the USB drivers are loaded, that forces devices to disconnect/reset,
> > > and they take a while to re-enumerate.  During that delay, the kernel gets
> > > to do_mount; now, no devices show in this "waiting for scan" count.

On Fri, Apr 25, 2008 at 09:30:52AM +0300, Pekka J Enberg wrote:
> > So how does that happen? ->storage_probe fails and driver core calls it 
> > later at some point?

On Fri, 25 Apr 2008, Matthew Dharm wrote:
> There's no guarantee that storage_probe is going to get called in a timely
> manner.

How can we add such a guarantee? Don't we have this problem with any other 
storage devices?

			Pekka

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Dharm]

[-- Attachment #1: Type: text/plain, Size: 1381 bytes --]

On Fri, Apr 25, 2008 at 11:19:02AM +0300, Pekka J Enberg wrote:
> Hi Matthew,
> 
> On Thu, 24 Apr 2008, Matthew Dharm wrote:
> > > > This also has all sorts of races between do_mounts 'waiting' and the actual
> > > > USB device enumeration.  It's entirely possible that the kernel loads via
> > > > BIOS, the USB drivers are loaded, that forces devices to disconnect/reset,
> > > > and they take a while to re-enumerate.  During that delay, the kernel gets
> > > > to do_mount; now, no devices show in this "waiting for scan" count.
> 
> On Fri, Apr 25, 2008 at 09:30:52AM +0300, Pekka J Enberg wrote:
> > > So how does that happen? ->storage_probe fails and driver core calls it 
> > > later at some point?
> 
> On Fri, 25 Apr 2008, Matthew Dharm wrote:
> > There's no guarantee that storage_probe is going to get called in a timely
> > manner.
> 
> How can we add such a guarantee? Don't we have this problem with any other 
> storage devices?

I doubt it.  It all depends on exactly when the USB controller initializes,
all relevant hubs get initialized, and the device finally discovered.

Matt

-- 
Matthew Dharm                              Home: mdharm-usb@one-eyed-alien.net 
Maintainer, Linux USB Mass Storage Driver

I don't have a left mouse button.  I only have one mouse and it's on my right.
					-- Customer
User Friendly, 2/13/1999

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Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Greg KH]

On Fri, Apr 25, 2008 at 11:19:02AM +0300, Pekka J Enberg wrote:
> Hi Matthew,
> 
> On Thu, 24 Apr 2008, Matthew Dharm wrote:
> > > > This also has all sorts of races between do_mounts 'waiting' and the actual
> > > > USB device enumeration.  It's entirely possible that the kernel loads via
> > > > BIOS, the USB drivers are loaded, that forces devices to disconnect/reset,
> > > > and they take a while to re-enumerate.  During that delay, the kernel gets
> > > > to do_mount; now, no devices show in this "waiting for scan" count.
> 
> On Fri, Apr 25, 2008 at 09:30:52AM +0300, Pekka J Enberg wrote:
> > > So how does that happen? ->storage_probe fails and driver core calls it 
> > > later at some point?
> 
> On Fri, 25 Apr 2008, Matthew Dharm wrote:
> > There's no guarantee that storage_probe is going to get called in a timely
> > manner.
> 
> How can we add such a guarantee? Don't we have this problem with any other 
> storage devices?

No we don't.  The problem with USB is that you _never_ know if you are
done discovering all of the devices that are currently plugged into the
bus.  For PCI SCSI and ATA devices, that is not an issue.  So no matter
how many times you think you can mark things "done" you never really
know for sure.

So this means that we can't do this in the kernel, use an initramfs if
you want such functionality, you can sit and spin there and wait until
the device that you think you "know" is there to show up before
continuing on with the boot process.

thanks,

greg k-h

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Pekka J Enberg]

On Thu, 24 Apr 2008, Greg KH wrote:
> What happens if there is no USB device present in the system?  Because
> of that, there is no way that this option could ever be enabled by any
> distro or anyone wanting their kernel to run on more than one machine :(

Not sure what you mean. If there are no USB devices present on the system, 
the boot proceeds as usual. The config option will make the kernel wait 
only if you have a USB mass storage device (or some other device that 
cause async SCSI scanning) _plugged in_.

		Pekka

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Stefan Richter]

(Adding Cc: Matthew Wilcox)

Pekka J Enberg wrote:
> Add a new kernel config option CONFIG_LATE_ROOT_MOUNT that makes the kernel
> wait until background scanning of USB mass storage devices is complete before
> attempting to mount the root filesystem.
> 
> The config option is an alternative to the root_delay= kernel parameter
> solution people currently use to boot from USB mass storage devices.
[...]
>  drivers/scsi/scsi_scan.c  |    2 ++
>  drivers/usb/storage/usb.c |    3 +++
>  include/linux/init.h      |   12 ++++++++++++
>  init/Kconfig              |    5 +++++
>  init/do_mounts.c          |   32 ++++++++++++++++++++++++++++++++
>  5 files changed, 54 insertions(+)
> 
> Index: linux-2.6/drivers/usb/storage/usb.c
> ===================================================================
> --- linux-2.6.orig/drivers/usb/storage/usb.c
> +++ linux-2.6/drivers/usb/storage/usb.c
> @@ -928,6 +928,7 @@ static int usb_stor_scan_thread(void * _
>  		/* Should we unbind if no devices were detected? */
>  	}
>  
> +	complete_root_scan();
>  	complete_and_exit(&us->scanning_done, 0);
>  }
>  
> @@ -1007,12 +1008,14 @@ static int storage_probe(struct usb_inte
>  		goto BadDevice;
>  	}
>  
> +	begin_root_scan();
>  	/* Start up the thread for delayed SCSI-device scanning */
>  	th = kthread_create(usb_stor_scan_thread, us, "usb-stor-scan");
>  	if (IS_ERR(th)) {
>  		printk(KERN_WARNING USB_STORAGE 
>  		       "Unable to start the device-scanning thread\n");
>  		quiesce_and_remove_host(us);
> +		complete_root_scan();
>  		result = PTR_ERR(th);
>  		goto BadDevice;
>  	}
[...]
> Index: linux-2.6/drivers/scsi/scsi_scan.c
> ===================================================================
> --- linux-2.6.orig/drivers/scsi/scsi_scan.c
> +++ linux-2.6/drivers/scsi/scsi_scan.c
> @@ -1802,6 +1802,7 @@ static int do_scan_async(void *_data)
>  	struct async_scan_data *data = _data;
>  	do_scsi_scan_host(data->shost);
>  	scsi_finish_async_scan(data);
> +	complete_root_scan();
>  	return 0;
>  }
>  
> @@ -1823,6 +1824,7 @@ void scsi_scan_host(struct Scsi_Host *sh
>  		return;
>  	}
>  
> +	begin_root_scan();
>  	p = kthread_run(do_scan_async, data, "scsi_scan_%d", shost->host_no);
>  	if (unlikely(IS_ERR(p)))
>  		do_scan_async(data);

Looks like you want complete_root_scan() after IS_ERR(p).  Are there any 
other thread exit paths which don't call complete_root_scan() yet?

Besides, can't you use the generic async scan facility of the SCSI stack 
for your purpose?  If not, can it be extended to make it usable?
-- 
Stefan Richter
-=====-==--- -=-- ==--=
http://arcgraph.de/sr/

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Pekka J Enberg]

Hi Stefan,

On Fri, 25 Apr 2008, Stefan Richter wrote:
> > Index: linux-2.6/drivers/scsi/scsi_scan.c
> > ===================================================================
> > --- linux-2.6.orig/drivers/scsi/scsi_scan.c
> > +++ linux-2.6/drivers/scsi/scsi_scan.c
> > @@ -1802,6 +1802,7 @@ static int do_scan_async(void *_data)
> >   struct async_scan_data *data = _data;
> >   do_scsi_scan_host(data->shost);
> >   scsi_finish_async_scan(data);
> > +	complete_root_scan();
> >  	return 0;
> >  }
> >  
> > @@ -1823,6 +1824,7 @@ void scsi_scan_host(struct Scsi_Host *sh
> >   	return;
> >   }
> > 
> > +	begin_root_scan();
> >   p = kthread_run(do_scan_async, data, "scsi_scan_%d", shost->host_no);
> >   if (unlikely(IS_ERR(p)))
> >    do_scan_async(data);
> 
> Looks like you want complete_root_scan() after IS_ERR(p).  Are there any other
> thread exit paths which don't call complete_root_scan() yet?

No, we call complete_root_scan() in do_scan_sync() which is either called 
asynchronously from the kernel thread or synchronously on IS_ERR(p).

On Fri, 25 Apr 2008, Stefan Richter wrote:
> Besides, can't you use the generic async scan facility of the SCSI stack for
> your purpose?  If not, can it be extended to make it usable?

So you mean drivers/scsi/scsi_scan.c, right? We probably can and 
should so I'll take a look.

		Pekka

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Wilcox]

On Fri, Apr 25, 2008 at 11:15:36AM +0300, Pekka J Enberg wrote:
> On Fri, 25 Apr 2008, Stefan Richter wrote:
> > Besides, can't you use the generic async scan facility of the SCSI stack for
> > your purpose?  If not, can it be extended to make it usable?
> 
> So you mean drivers/scsi/scsi_scan.c, right? We probably can and 
> should so I'll take a look.

The problem is that USB has one scsi_host per device (rather than, say,
having one scsi_host and adding new devices to it as they're found on
the USB bus).

I'm certainly open to ideas of hooking into the mechanism -- it's just a
list of completions after all.  Something like this:

void *scsi_insert_into_scan_list(void)
{
	return scsi_prep_async_scan(NULL);
}

void scsi_scan_completed(void *cookie)
{
	struct async_scan_data *data = cookie;
	wait_for_completion(&data->prev_finished);
	spin_lock(&async_scan_lock);
	list_del(&data->list);
	if (!list_empty(&scanning_hosts)) {
		struct async_scan_data *next = list_entry(scanning_hosts.next,
				struct async_scan_data, list);
		complete(&next->prev_finished);
	}
	spin_unlock(&async_scan_lock);

	kfree(data);
}

Perhaps refactor a little -- everything inside the async_scan_lock would
be common to scsi_scan_completed() and scsi_finish_async_scan().

with appropriate checks for shost being NULL in scsi_prep_async_scan:

+++ b/drivers/scsi/scsi_scan.c
@@ -1698,7 +1698,7 @@ static struct async_scan_data *scsi_prep_async_scan(struct
        if (strncmp(scsi_scan_type, "sync", 4) == 0)
                return NULL;
 
-       if (shost->async_scan) {
+       if (shost && shost->async_scan) {
                printk("%s called twice for host %d", __FUNCTION__,
                                shost->host_no);
                dump_stack();
@@ -1708,16 +1708,19 @@ static struct async_scan_data *scsi_prep_async_scan(stru
        data = kmalloc(sizeof(*data), GFP_KERNEL);
        if (!data)
                goto err;
-       data->shost = scsi_host_get(shost);
-       if (!data->shost)
-               goto err;
        init_completion(&data->prev_finished);
 
-       mutex_lock(&shost->scan_mutex);
-       spin_lock_irqsave(shost->host_lock, flags);
-       shost->async_scan = 1;
-       spin_unlock_irqrestore(shost->host_lock, flags);
-       mutex_unlock(&shost->scan_mutex);
+       if (shost) {
+               data->shost = scsi_host_get(shost);
+               if (!data->shost)
+                       goto err;
+
+               mutex_lock(&shost->scan_mutex);
+               spin_lock_irqsave(shost->host_lock, flags);
+               shost->async_scan = 1;
+               spin_unlock_irqrestore(shost->host_lock, flags);
+               mutex_unlock(&shost->scan_mutex);
+       }
 
        spin_lock(&async_scan_lock);
        if (list_empty(&scanning_hosts))

Would that work for you?

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Wilcox]

This discussion seemed to die off ... did anything ever come of it?

On Fri, Apr 25, 2008 at 03:11:23AM -0600, Matthew Wilcox wrote:
> On Fri, Apr 25, 2008 at 11:15:36AM +0300, Pekka J Enberg wrote:
> > On Fri, 25 Apr 2008, Stefan Richter wrote:
> > > Besides, can't you use the generic async scan facility of the SCSI stack for
> > > your purpose?  If not, can it be extended to make it usable?
> > 
> > So you mean drivers/scsi/scsi_scan.c, right? We probably can and 
> > should so I'll take a look.
> 
> The problem is that USB has one scsi_host per device (rather than, say,
> having one scsi_host and adding new devices to it as they're found on
> the USB bus).
> 
> I'm certainly open to ideas of hooking into the mechanism -- it's just a
> list of completions after all.  Something like this:
> 
> void *scsi_insert_into_scan_list(void)
> {
> 	return scsi_prep_async_scan(NULL);
> }
> 
> void scsi_scan_completed(void *cookie)
> {
> 	struct async_scan_data *data = cookie;
> 	wait_for_completion(&data->prev_finished);
> 	spin_lock(&async_scan_lock);
> 	list_del(&data->list);
> 	if (!list_empty(&scanning_hosts)) {
> 		struct async_scan_data *next = list_entry(scanning_hosts.next,
> 				struct async_scan_data, list);
> 		complete(&next->prev_finished);
> 	}
> 	spin_unlock(&async_scan_lock);
> 
> 	kfree(data);
> }
> 
> Perhaps refactor a little -- everything inside the async_scan_lock would
> be common to scsi_scan_completed() and scsi_finish_async_scan().
> 
> with appropriate checks for shost being NULL in scsi_prep_async_scan:
> 
> +++ b/drivers/scsi/scsi_scan.c
> @@ -1698,7 +1698,7 @@ static struct async_scan_data *scsi_prep_async_scan(struct
>         if (strncmp(scsi_scan_type, "sync", 4) == 0)
>                 return NULL;
>  
> -       if (shost->async_scan) {
> +       if (shost && shost->async_scan) {
>                 printk("%s called twice for host %d", __FUNCTION__,
>                                 shost->host_no);
>                 dump_stack();
> @@ -1708,16 +1708,19 @@ static struct async_scan_data *scsi_prep_async_scan(stru
>         data = kmalloc(sizeof(*data), GFP_KERNEL);
>         if (!data)
>                 goto err;
> -       data->shost = scsi_host_get(shost);
> -       if (!data->shost)
> -               goto err;
>         init_completion(&data->prev_finished);
>  
> -       mutex_lock(&shost->scan_mutex);
> -       spin_lock_irqsave(shost->host_lock, flags);
> -       shost->async_scan = 1;
> -       spin_unlock_irqrestore(shost->host_lock, flags);
> -       mutex_unlock(&shost->scan_mutex);
> +       if (shost) {
> +               data->shost = scsi_host_get(shost);
> +               if (!data->shost)
> +                       goto err;
> +
> +               mutex_lock(&shost->scan_mutex);
> +               spin_lock_irqsave(shost->host_lock, flags);
> +               shost->async_scan = 1;
> +               spin_unlock_irqrestore(shost->host_lock, flags);
> +               mutex_unlock(&shost->scan_mutex);
> +       }
>  
>         spin_lock(&async_scan_lock);
>         if (list_empty(&scanning_hosts))
> 
> Would that work for you?
> 
> -- 
> Intel are signing my paycheques ... these opinions are still mine
> "Bill, look, we understand that you're interested in selling us this
> operating system, but compare it to ours.  We can't possibly take such
> a retrograde step."

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Stefan Richter]

> On Fri, Apr 25, 2008 at 03:11:23AM -0600, Matthew Wilcox wrote:
>> The problem is that USB has one scsi_host per device (rather than, say,
>> having one scsi_host and adding new devices to it as they're found on
>> the USB bus).
>>
>> I'm certainly open to ideas of hooking into the mechanism -- it's just a
>> list of completions after all.  Something like this:
[...]

The problem is AFAIU not exactly how many Scsi_Host instances (initiator 
port representations, or something remotely related to that) are 
instantiated, but rather _when_ they are instantiated.  On multi 
protocol buses or networks like USB and FireWire, it makes sense to 
create the SCSI initiator port representation when we discovered that 
there are actually SCSI targets on the bus or network.

scsi_scan's API is geared towards drivers which create one or more 
initiator port representations right up front before going on target 
discovery, right?
-- 
Stefan Richter
-=====-==--- -==- =--==
http://arcgraph.de/sr/

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Stefan Richter]

Stefan Richter wrote:
> The problem is AFAIU not exactly how many Scsi_Host instances (initiator 
> port representations, or something remotely related to that) are 
> instantiated, but rather _when_ they are instantiated.  On multi 
> protocol buses or networks like USB and FireWire, it makes sense to 
> create the SCSI initiator port representation when we discovered that 
> there are actually SCSI targets on the bus or network.
> 
> scsi_scan's API

scsi_wait_scan's API

> is geared towards drivers which create one or more 
> initiator port representations right up front before going on target 
> discovery, right?
-- 
Stefan Richter
-=====-==--- -==- =--==
http://arcgraph.de/sr/

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Wilcox]

On Thu, Jun 19, 2008 at 08:39:38PM +0200, Stefan Richter wrote:
> The problem is AFAIU not exactly how many Scsi_Host instances (initiator 
> port representations, or something remotely related to that) are 
> instantiated, but rather _when_ they are instantiated.  On multi 
> protocol buses or networks like USB and FireWire, it makes sense to 
> create the SCSI initiator port representation when we discovered that 
> there are actually SCSI targets on the bus or network.

I'm not convinced of that.  Why shouldn't we create one scsi host for
all USB scsi devices?  I know that today there's a certain amount of
per-device state stored in the scsi_host, but that should be fixable.

> scsi_scan's API is geared towards drivers which create one or more 
> initiator port representations right up front before going on target 
> discovery, right?

Today, yes.  The mail that I just pinged described a way to enhance the
API -- letting you tell scsi "I've started scanning, save me a place in
the queue" and "I've finished scanning, hook me up".

The advantage to this is that you can spend a lot more time waiting for
devices without increasing boot time significantly, since you wait while
the rest of the system initialises.

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Dharm]

[-- Attachment #1: Type: text/plain, Size: 1308 bytes --]

On Thu, Jun 19, 2008 at 12:54:19PM -0600, Matthew Wilcox wrote:
> On Thu, Jun 19, 2008 at 08:39:38PM +0200, Stefan Richter wrote:
> > The problem is AFAIU not exactly how many Scsi_Host instances (initiator 
> > port representations, or something remotely related to that) are 
> > instantiated, but rather _when_ they are instantiated.  On multi 
> > protocol buses or networks like USB and FireWire, it makes sense to 
> > create the SCSI initiator port representation when we discovered that 
> > there are actually SCSI targets on the bus or network.
> 
> I'm not convinced of that.  Why shouldn't we create one scsi host for
> all USB scsi devices?  I know that today there's a certain amount of
> per-device state stored in the scsi_host, but that should be fixable.

First, there are usb-storage devices which are USB<->SCSI bridges and
therefore support multiple target devices.

Second, you would still need multiple hosts when you have more than 15
target USB devices, anyway.

Matt

-- 
Matthew Dharm                              Home: mdharm-usb@one-eyed-alien.net 
Maintainer, Linux USB Mass Storage Driver

E:  You run this ship with Windows?!  YOU IDIOT!
L:  Give me a break, it came bundled with the computer!
					-- ESR and Lan Solaris
User Friendly, 12/8/1998

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Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Wilcox]

On Thu, Jun 19, 2008 at 12:52:40PM -0700, Matthew Dharm wrote:
> First, there are usb-storage devices which are USB<->SCSI bridges and
> therefore support multiple target devices.

This is true.

> Second, you would still need multiple hosts when you have more than 15
> target USB devices, anyway.

This isn't true.  Look at topologies like fibre channel which can have
thousands of targets on a single host.

I would suggest that you want one 'channel' per usb endpoint.

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Stefan Richter]

Let's face it, from the POV of several if not all transport layer 
drivers, the core API element Scsi_Host is just a superfluous appendage 
to the only interesting elements -- target and logical unit, or even 
logical unit alone.

Matthew Wilcox wrote:
> I would suggest that you want one 'channel' per usb endpoint.

No, "channel" is meaningless.  Friends don't let friends invent new 
meanings for "channel".
-- 
Stefan Richter
-=====-==--- -==- =--==
http://arcgraph.de/sr/

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Stefan Richter]

Matthew Wilcox wrote:
> On Thu, Jun 19, 2008 at 08:39:38PM +0200, Stefan Richter wrote:
>> The problem is AFAIU not exactly how many Scsi_Host instances (initiator 
>> port representations, or something remotely related to that) are 
>> instantiated, but rather _when_ they are instantiated.  On multi 
>> protocol buses or networks like USB and FireWire, it makes sense to 
>> create the SCSI initiator port representation when we discovered that 
>> there are actually SCSI targets on the bus or network.
> 
> I'm not convinced of that.

Drivers like usb-storage and sbp2 are high-level drivers (protocol
drivers) on top of driver stacks.  They a priori talk to struct device
instances which represent subunits on "external" devices; they don't
have much business with the local host controller device.  For them,
"SCSI initiator port" is a rather hazy and, in the implementation,
uninteresting concept.  (That's how it is with sbp2; I'm only guessing
how it is with usb-storage.)

However, there are several ways to accomplish the same thing in the
implementation.  I heard there were more constraints under Linux 2.4;
some current implementation details may just be legacy.

> Why shouldn't we create one scsi host for
> all USB scsi devices?

Or one for each USB HCD.

I thought about that for FireWire but never started to actually try it
out.  Parent-child relationships of struct devices would be different
from what we have today.

[...]
> The mail that I just pinged described a way to enhance the
> API -- letting you tell scsi "I've started scanning, save me a place in
> the queue" and "I've finished scanning, hook me up".

When would be the right time to say "I've started scanning"?  SCSI
transport driver initialization?  Or end of USB/FireWire/... HCD
registration?

When to say "I've finished scanning"? --- I think the answer is:  When
(a) probing of a configurable number of SCSI targets or logical units
was finished or (b) a configurable timeout occurs, whatever of the two
happens first.

(Explanation, if one is necessary:  SCSI targets may pop out of the
woods any time; it is impossible to say "from this moment on there won't
show up any new ones anymore".  This is just as true for cold boot and
warm boot as it is for subsequent operation.)
-- 
Stefan Richter
-=====-==--- -==- =--==
http://arcgraph.de/sr/

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Wilcox]

On Thu, Jun 19, 2008 at 09:54:06PM +0200, Stefan Richter wrote:
> [...]
> > The mail that I just pinged described a way to enhance the
> > API -- letting you tell scsi "I've started scanning, save me a place in
> > the queue" and "I've finished scanning, hook me up".
> 
> When would be the right time to say "I've started scanning"?  SCSI
> transport driver initialization?  Or end of USB/FireWire/... HCD
> registration?

You would need to call it before you could attempt to register any
devices.  So fairly early on in the initialisation of sbp2/usb-storage.

> When to say "I've finished scanning"? --- I think the answer is:  When
> (a) probing of a configurable number of SCSI targets or logical units
> was finished or (b) a configurable timeout occurs, whatever of the two
> happens first.
> 
> (Explanation, if one is necessary:  SCSI targets may pop out of the
> woods any time; it is impossible to say "from this moment on there won't
> show up any new ones anymore".  This is just as true for cold boot and
> warm boot as it is for subsequent operation.)

Yes, I do see the problem.  There's no way the SCSI core can know when
a driver has finished scanning, so I've punted to the driver to set its
own timeout here.  I know that new devices really can show up at any
time, but I think it's reasonable to say that if a machine is booted
with the same configuration as last time, the drives should show up with
the same names.

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Stefan Richter]

Matthew Wilcox wrote:
> On Thu, Jun 19, 2008 at 09:54:06PM +0200, Stefan Richter wrote:
>> When to say "I've finished scanning"? --- I think the answer is:  When
>> (a) probing of a configurable number of SCSI targets or logical units
>> was finished or (b) a configurable timeout occurs, whatever of the two
>> happens first.
...
> Yes, I do see the problem.  There's no way the SCSI core can know when
> a driver has finished scanning, so I've punted to the driver to set its
> own timeout here.

This can of course (and frequently is) implemented in userspace, but
requires an initrd.  I'm not up to speed how scsi_wait_scan is put to
use --- it can be used without initrd, can't it?

> I know that new devices really can show up at any
> time, but I think it's reasonable to say that if a machine is booted
> with the same configuration as last time, the drives should show up with
> the same names.

I know that is not what you meant here, but...:

They do show up with the same target identifiers and logical unit
identifiers.  The SCSI core doesn't pass the identifiers through to
userland though.  (Couldn't resist to go off-subject.  Sorry.)
-- 
Stefan Richter
-=====-==--- -==- =--==
http://arcgraph.de/sr/

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Alan Stern]

On Thu, 19 Jun 2008, Matthew Wilcox wrote:

> This discussion seemed to die off ... did anything ever come of it?

As I recall, it died because the whole notion was very poorly defined
to begin with.  The idea was to stop waiting when all the SCSI buses
had been scanned -- but there's no way to know when that occurs because
new buses can be added at any time.

Maybe a better approach would be to poll at reasonable intervals 
(HZ/10, for example) for the existence of the root device.  If it 
hasn't appeared after some reasonable time (30 seconds?) then give up.

Alan Stern

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[David Brownell]

On Thursday 19 June 2008, Alan Stern wrote:
> Maybe a better approach would be to poll at reasonable intervals 
> (HZ/10, for example) for the existence of the root device.  If it 
> hasn't appeared after some reasonable time (30 seconds?) then give up.

Why give up, ever? 

If someone has to walk across the room to grab the device with the
root filesystem, then has to answer the phone, then deal with the
next interruption ... for an hour before getting back to the machine
they rebooted, why shouldn't it just work to plug in the USB flash
drive (CF card, MMC card, or whatever) *THEN* instead of having had
to rush and meet some arbitrarily early deadline?

When there's no root, I don't see how waiting for it to arrive
could be a lose...

- Dave

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Gabor Gombas]

On Thu, Jun 19, 2008 at 04:13:25PM -0700, David Brownell wrote:

> Why give up, ever? 

It would be quite useful to time out and reboot using the previous
kernel ("lilo -R") when one wants to figure out remotely why the new
kernel does not boot.

Gabor

-- 
     ---------------------------------------------------------
     MTA SZTAKI Computer and Automation Research Institute
                Hungarian Academy of Sciences
     ---------------------------------------------------------

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Dharm]

[-- Attachment #1: Type: text/plain, Size: 761 bytes --]

On Fri, Jun 20, 2008 at 11:53:11AM +0200, Gabor Gombas wrote:
> On Thu, Jun 19, 2008 at 04:13:25PM -0700, David Brownell wrote:
> 
> > Why give up, ever? 
> 
> It would be quite useful to time out and reboot using the previous
> kernel ("lilo -R") when one wants to figure out remotely why the new
> kernel does not boot.

And thus we arrive at only one conclusion:  You should do this with an
initrd so that any policy that is desired can be implemented.  No matter
what the kernel does by default, it will be incorrect for some scenarios.

Matt

-- 
Matthew Dharm                              Home: mdharm-usb@one-eyed-alien.net 
Maintainer, Linux USB Mass Storage Driver

It was a new hope.
					-- Dust Puppy
User Friendly, 12/25/1998

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Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Vegard Nossum]

Hi,

On Thu, Jun 19, 2008 at 11:39 PM, Alan Stern <stern@rowland.harvard.edu> wrote:
> On Thu, 19 Jun 2008, Matthew Wilcox wrote:
>
>> This discussion seemed to die off ... did anything ever come of it?
>
> As I recall, it died because the whole notion was very poorly defined
> to begin with.  The idea was to stop waiting when all the SCSI buses
> had been scanned -- but there's no way to know when that occurs because
> new buses can be added at any time.

Can you please explain why this is?

We only want to scan buses that are present when the system is
started. If we reach the point where all USB ports/devices/whatever
have been enumerated, all SCSI buses scanned, and all partition tables
loaded, what more is there to wait for?

I can't understand that this is fundamentally a hardware problem. I
understand that there might be a problem with the patch that was
proposed a the beginning of the thread, but is this really a truly
unsolvable problem? Please correct me if I am wrong -- the problem
here is that Pekka's newly introduced nr_root_scans can drop to 0
before everything has been enumerated at least once. This is because
scsi_scan_host() forks a new thread which is what's actually doing the
scanning. Can't we just stick a begin_root_scan() before forking, and
drop it inside the thread, just like we do with the
usb_stor_scan_thread? If the thread is actually a loop, the first
iteration should be enough, right?

I'm grateful for any explanations that will help my poor head understand... :-)


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Matthew Wilcox]

On Fri, Jun 20, 2008 at 05:15:43PM +0200, Vegard Nossum wrote:
> On Thu, Jun 19, 2008 at 11:39 PM, Alan Stern <stern@rowland.harvard.edu> wrote:
> > As I recall, it died because the whole notion was very poorly defined
> > to begin with.  The idea was to stop waiting when all the SCSI buses
> > had been scanned -- but there's no way to know when that occurs because
> > new buses can be added at any time.
> 
> Can you please explain why this is?

The USB spec doesn't mandate a minimum time before devices come
available.  There's no way for the device to say "I'm here, I will be
providing USB Storage, but I'm not ready yet".  So we can't know when
we've enumerated all the USB devices on the system.


-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Alan Stern]

On Fri, 20 Jun 2008, Vegard Nossum wrote:

> Hi,
> 
> On Thu, Jun 19, 2008 at 11:39 PM, Alan Stern <stern@rowland.harvard.edu> wrote:
> > On Thu, 19 Jun 2008, Matthew Wilcox wrote:
> >
> >> This discussion seemed to die off ... did anything ever come of it?
> >
> > As I recall, it died because the whole notion was very poorly defined
> > to begin with.  The idea was to stop waiting when all the SCSI buses
> > had been scanned -- but there's no way to know when that occurs because
> > new buses can be added at any time.
> 
> Can you please explain why this is?
> 
> We only want to scan buses that are present when the system is
> started. If we reach the point where all USB ports/devices/whatever
> have been enumerated, all SCSI buses scanned, and all partition tables
> loaded, what more is there to wait for?

New USB devices can appear at any time.  None are present when the 
system is started; they are all detected dynamically by the khubd 
thread.

While I'm not familiar with the details of any other hotpluggable 
buses (like Firewire), I imagine much the same is true for them.

> I can't understand that this is fundamentally a hardware problem. I
> understand that there might be a problem with the patch that was
> proposed a the beginning of the thread, but is this really a truly
> unsolvable problem? Please correct me if I am wrong -- the problem
> here is that Pekka's newly introduced nr_root_scans can drop to 0
> before everything has been enumerated at least once. This is because
> scsi_scan_host() forks a new thread which is what's actually doing the
> scanning. Can't we just stick a begin_root_scan() before forking, and
> drop it inside the thread, just like we do with the
> usb_stor_scan_thread? If the thread is actually a loop, the first
> iteration should be enough, right?
> 
> I'm grateful for any explanations that will help my poor head understand... :-)

Suppose there are two USB disk drives attached, and the one containing
the root fs is probed second.  The count could easily drop to 0 when
the first drive has been scanned, especially if scanning is done
synchronously.

What happens when the root fs is on a hotpluggable bus that doesn't use
the SCSI infrastructure?  You'd have to extend your hooks into that
other subsystem as well.

Besides, why do you want to go to all this trouble to find out
_exactly_ when the device containing the root fs becomes available?  
What's wrong with just checking at 1/10-second intervals?

For example, what happens if the USB device containing the root fs
isn't plugged in when the system boots?  The user will quickly realize
that nothing is happening and will then plug it in.  But your code will
already have decided that the device doesn't exist and given up.  
Wouldn't it be better simply to keep on trying, so that then the device 
does get plugged in the system can use it?

Alan Stern

Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

[Stefan Richter]

Alan Stern wrote:
> New USB devices can appear at any time.  None are present when the 
> system is started; they are all detected dynamically by the khubd 
> thread.
> 
> While I'm not familiar with the details of any other hotpluggable 
> buses (like Firewire), I imagine much the same is true for them.

It's very similar with FireWire.  FireWire devices announce SBP-2 target
capability (if they have it) after they became ready.  The ieee1394 or
firewire core then scans or re-scans them in a process context.  Then
upper layer drivers like the SBP-2 driver are matched, bound, probed.

If SBP-2 targets went through a power state change, they typically first
power everything up (spin up media etc.) before they announce their
protocol capabilities, and this may of course take a while.  Before that
they just are inaccessible, and it is unknown whether they could become
candidates for access by upper layer drivers.
-- 
Stefan Richter
-=====-==--- -==- =-=--
http://arcgraph.de/sr/