[RFC]mooring API

[Oliver Neukum <oneukum@suse.com>]

Hi,

why this new API? Eli found a race with the existing API. Many
drivers are acribing to it semantics it never had. Now we have
sort of a fix, but it is not really elegant.
The anchors have always been about making it easier to write drivers.
Hence if driver authors are assumuning that they have a power, we
better provide that facility. What users need is a facility
to group URBs together and get rid of them no questions asked.
It would be best if we can do that with minimal changes.

Here is a V2 taking into account Alan's remarks, and using a separate
flag.

	Regards
		Oliver

From 79df4240287b712bbe08404af7f900c3bccfca40 Mon Sep 17 00:00:00 2001
From: Oliver Neukum <oneukum@suse.com>
Date: Tue, 28 Jul 2020 11:38:23 +0200
Subject: [PATCH] USB: add mooring API

This is a simplified and thereby better version of the anchor API.
Anchors have the problem that they unanchor an URB upon giveback,
which creates a window during which an URB is unanchored but not
yet returned, leading to operations on anchors not having the
semantics many driver errornously assume them to have.
The new API keeps an URB on an anchor until it is explicitly
unmoored.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
---
 drivers/usb/core/hcd.c |  4 +++-
 drivers/usb/core/urb.c | 27 ++++++++++++++++++++++++++-
 include/linux/usb.h    |  3 +++
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index a33b849e8beb..e1d26cb595c3 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1640,7 +1640,9 @@ static void __usb_hcd_giveback_urb(struct urb *urb)
 	unmap_urb_for_dma(hcd, urb);
 	usbmon_urb_complete(&hcd->self, urb, status);
 	usb_anchor_suspend_wakeups(anchor);
-	usb_unanchor_urb(urb);
+	smp_rmb(); /* against usb_(un)moor_urb() */
+	if (!urb->moored)
+		usb_unanchor_urb(urb);
 	if (likely(status == 0))
 		usb_led_activity(USB_LED_EVENT_HOST);
 
diff --git a/drivers/usb/core/urb.c b/drivers/usb/core/urb.c
index 7bc23469f4e4..ee3c6c7c2630 100644
--- a/drivers/usb/core/urb.c
+++ b/drivers/usb/core/urb.c
@@ -123,7 +123,7 @@ EXPORT_SYMBOL_GPL(usb_get_urb);
  * This can be called to have access to URBs which are to be executed
  * without bothering to track them
  */
-void usb_anchor_urb(struct urb *urb, struct usb_anchor *anchor)
+static void __usb_anchor_urb(struct urb *urb, struct usb_anchor *anchor)
 {
 	unsigned long flags;
 
@@ -137,8 +137,20 @@ void usb_anchor_urb(struct urb *urb, struct usb_anchor *anchor)
 
 	spin_unlock_irqrestore(&anchor->lock, flags);
 }
+
+void usb_anchor_urb(struct urb *urb, struct usb_anchor *anchor)
+{
+	__usb_anchor_urb(urb, anchor);
+}
 EXPORT_SYMBOL_GPL(usb_anchor_urb);
 
+void usb_moor_urb(struct urb *urb, struct usb_anchor *anchor)
+{
+	urb->moored = true;
+	__usb_anchor_urb(urb, anchor);
+}
+EXPORT_SYMBOL_GPL(usb_moor_urb);
+
 static int usb_anchor_check_wakeup(struct usb_anchor *anchor)
 {
 	return atomic_read(&anchor->suspend_wakeups) == 0 &&
@@ -185,6 +197,19 @@ void usb_unanchor_urb(struct urb *urb)
 }
 EXPORT_SYMBOL_GPL(usb_unanchor_urb);
 
+void usb_unmoor_urb(struct urb *urb)
+{
+	struct usb_anchor *anchor;
+
+	urb->moored = false;
+	anchor = urb->anchor;
+	if (!anchor)
+		return;
+
+	__usb_unanchor_urb(urb, anchor);
+}
+EXPORT_SYMBOL_GPL(usb_unmoor_urb);
+
 /*-------------------------------------------------------------------*/
 
 static const int pipetypes[4] = {
diff --git a/include/linux/usb.h b/include/linux/usb.h
index 20c555db4621..b9e1464a2552 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -1565,6 +1565,7 @@ struct urb {
 	void *hcpriv;			/* private data for host controller */
 	atomic_t use_count;		/* concurrent submissions counter */
 	atomic_t reject;		/* submissions will fail */
+	bool moored;			/* the URB is moored not anchored */
 
 	/* public: documented fields in the urb that can be used by drivers */
 	struct list_head urb_list;	/* list head for use by the urb's
@@ -1732,6 +1733,8 @@ extern void usb_anchor_suspend_wakeups(struct usb_anchor *anchor);
 extern void usb_anchor_resume_wakeups(struct usb_anchor *anchor);
 extern void usb_anchor_urb(struct urb *urb, struct usb_anchor *anchor);
 extern void usb_unanchor_urb(struct urb *urb);
+extern void usb_moor_urb(struct urb *urb, struct usb_anchor *anchor);
+extern void usb_unmoor_urb(struct urb *urb);
 extern int usb_wait_anchor_empty_timeout(struct usb_anchor *anchor,
 					 unsigned int timeout);
 extern struct urb *usb_get_from_anchor(struct usb_anchor *anchor);
-- 
2.16.4