[PATCH] ATA over Ethernet: Convert emsgs_sema in a completion

[PATCH] ATA over Ethernet: Convert emsgs_sema in a completion

[Matthias Kaehlcke]

ATA over Ethernet: The semaphore emsgs_sema is used for signaling an
event, convert it in a completion.

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>

--

diff --git a/drivers/block/aoe/aoechr.c b/drivers/block/aoe/aoechr.c
index e8e60e7..d0ffe63 100644
--- a/drivers/block/aoe/aoechr.c
+++ b/drivers/block/aoe/aoechr.c
@@ -6,6 +6,7 @@
 
 #include <linux/hdreg.h>
 #include <linux/blkdev.h>
+#include <linux/completion.h>
 #include <linux/delay.h>
 #include "aoe.h"
 
@@ -35,7 +36,7 @@ struct ErrMsg {
 
 static struct ErrMsg emsgs[NMSG];
 static int emsgs_head_idx, emsgs_tail_idx;
-static struct semaphore emsgs_sema;
+static struct completion emsgs_comp;
 static spinlock_t emsgs_lock;
 static int nblocked_emsgs_readers;
 static struct class *aoe_class;
@@ -140,7 +141,7 @@ bail:		spin_unlock_irqrestore(&emsgs_lock, flags);
 	spin_unlock_irqrestore(&emsgs_lock, flags);
 
 	if (nblocked_emsgs_readers)
-		up(&emsgs_sema);
+		complete(&emsgs_comp);
 }
 
 static ssize_t
@@ -216,7 +217,7 @@ aoechr_read(struct file *filp, char __user *buf, size_t cnt, loff_t *off)
 
 		spin_unlock_irqrestore(&emsgs_lock, flags);
 
-		n = down_interruptible(&emsgs_sema);
+		n = wait_for_completion_interruptible(&emsgs_comp);
 
 		spin_lock_irqsave(&emsgs_lock, flags);
 
@@ -264,7 +265,7 @@ aoechr_init(void)
 		printk(KERN_ERR "aoe: can't register char device\n");
 		return n;
 	}
-	sema_init(&emsgs_sema, 0);
+	init_completion(&emsgs_comp);
 	spin_lock_init(&emsgs_lock);
 	aoe_class = class_create(THIS_MODULE, "aoe");
 	if (IS_ERR(aoe_class)) {

-- 
Matthias Kaehlcke
Embedded Linux Engineer
Barcelona

             You must have a plan. If you don't have a plan,
               you'll become part of somebody else's plan
                                                                 .''`.
    using free software / Debian GNU/Linux | http://debian.org  : :'  :
                                                                `. `'`
gpg --keyserver pgp.mit.edu --recv-keys 47D8E5D4                  `-

Re: [PATCH] ATA over Ethernet: Convert emsgs_sema in a completion

[Andrew Morton]

On Fri, 16 May 2008 21:11:33 +0200
Matthias Kaehlcke <matthias@kaehlcke.net> wrote:

> @@ -216,7 +217,7 @@ aoechr_read(struct file *filp, char __user *buf, size_t cnt, loff_t *off)
>  
>  		spin_unlock_irqrestore(&emsgs_lock, flags);
>  
> -		n = down_interruptible(&emsgs_sema);
> +		n = wait_for_completion_interruptible(&emsgs_comp);

Patch looks OK, but I worry about this code.  Suppose a signal is sent
and we return to userspace with the complete() still outstanding.  Then
someone removes the module.  

Perhaps there is synchronisation code which prevents that crash?

Re: [PATCH] ATA over Ethernet: Convert emsgs_sema in a completion

[Ed L. Cashin]

On Mon, May 19, 2008 at 04:51:22PM -0700, Andrew Morton wrote:
> On Fri, 16 May 2008 21:11:33 +0200
> Matthias Kaehlcke <matthias@kaehlcke.net> wrote:
> 
> > @@ -216,7 +217,7 @@ aoechr_read(struct file *filp, char __user *buf, size_t cnt, loff_t *off)
> >  
> >  		spin_unlock_irqrestore(&emsgs_lock, flags);
> >  
> > -		n = down_interruptible(&emsgs_sema);
> > +		n = wait_for_completion_interruptible(&emsgs_comp);
> 
> Patch looks OK, but I worry about this code.  Suppose a signal is sent
> and we return to userspace with the complete() still outstanding.  Then
> someone removes the module.  
> 
> Perhaps there is synchronisation code which prevents that crash?

I plan to answer this question soon.  Right now I am quite busy.

This emsgs_sema part of the aoe driver has never caused any problems
for users but has caused many problems for me, because it is tricky to
think about, to talk about, and for others to patch.

I hope that there is a more straightforward way to get the current
read-from-the-file-forever behavior using an existing kernel
facility.  A user currently can do,

  cat /dev/etherd/err

... and have it block, waiting until there is an error to report,
printing it, and then blocking again, like reading from a pipe.

-- 
  Ed L Cashin <ecashin@coraid.com>

Re: [PATCH] ATA over Ethernet: Convert emsgs_sema in a completion

[Andrew Morton]

On Wed, 21 May 2008 10:49:24 -0400 "Ed L. Cashin" <ecashin@coraid.com> wrote:

> On Mon, May 19, 2008 at 04:51:22PM -0700, Andrew Morton wrote:
> > On Fri, 16 May 2008 21:11:33 +0200
> > Matthias Kaehlcke <matthias@kaehlcke.net> wrote:
> > 
> > > @@ -216,7 +217,7 @@ aoechr_read(struct file *filp, char __user *buf, size_t cnt, loff_t *off)
> > >  
> > >  		spin_unlock_irqrestore(&emsgs_lock, flags);
> > >  
> > > -		n = down_interruptible(&emsgs_sema);
> > > +		n = wait_for_completion_interruptible(&emsgs_comp);
> > 
> > Patch looks OK, but I worry about this code.  Suppose a signal is sent
> > and we return to userspace with the complete() still outstanding.  Then
> > someone removes the module.  
> > 
> > Perhaps there is synchronisation code which prevents that crash?
> 
> I plan to answer this question soon.  Right now I am quite busy.
> 
> This emsgs_sema part of the aoe driver has never caused any problems
> for users but has caused many problems for me, because it is tricky to
> think about, to talk about, and for others to patch.
> 
> I hope that there is a more straightforward way to get the current
> read-from-the-file-forever behavior using an existing kernel
> facility.  A user currently can do,
> 
>   cat /dev/etherd/err
> 
> ... and have it block, waiting until there is an error to report,
> printing it, and then blocking again, like reading from a pipe.

That's fine - the normal way of handling that is for the user's fd to
hold a reference against the module itself.