Re: A reliable kernel panic (3.6.2) and system crash when visiting a particular website

[Takashi Iwai <tiwai@suse.de>]

At Wed, 7 Nov 2012 12:34:43 -0500 (EST),
Alan Stern wrote:
> 
> On Mon, 5 Nov 2012, Christof Meerwald wrote:
> 
> > BTW, I have been able to reproduce the problem on a completely
> > different machine (also running Ubuntu 12.10, but different hardware).
> > The important thing appears to be that the USB audio device is
> > connected via a USB 2.0 hub (and then using the test code posted in
> > http://pastebin.com/aHGe1S1X specifying the audio device as
> > "plughw:Set" (or whatever it's called) seems to trigger the freeze).
> 
> Christof: Thank you for that reference, it was a big help.  After
> crashing my system many times I have tracked the problem, at least in
> part.  The patch below should prevent your system from freezing.
> 
> 
> Takashi: It turns out the the problem is triggered when the audio
> subsystem calls snd_usb_endpoint_stop() with wait == 0 and then calls
> snd_usb_endpoint_start().  Since the driver doesn't wait for the
> outstanding URBs to finish, it tries to submit them again while they
> are still active.
> 
> Normally the USB core would realize this and fail the submission, but a
> bug in ehci-hcd prevented this from happening.  (That bug is what the
> patch below fixes.)  The URB gets added to the active list twice,
> resulting in list corruption and an oops in interrupt context, which
> freezes the system.
> 
> The user program that triggers the problem basically looks like this:
> 
>   snd_pcm_prepare(rec_pcm);
>   snd_pcm_start(rec_pcm);
>   snd_pcm_drop(rec_pcm);
> 
>   snd_pcm_prepare(rec_pcm);
>   snd_pcm_start(rec_pcm);
> 
> The snd_pcm_drop call unlinks the URBs but does not wait for them to
> finish.  Then the second snd_pcm_start call submits the URBs before
> they have finished.
> 
> What is the right solution for this problem?

How about the patch below?  (It's for 3.6, and won't be applied cleanly
to 3.7, but easy to adapt.)


Takashi

---
diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index d9de667..38830e2 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -35,6 +35,7 @@
 
 #define EP_FLAG_ACTIVATED	0
 #define EP_FLAG_RUNNING		1
+#define EP_FLAG_STOPPING	2
 
 /*
  * snd_usb_endpoint is a model that abstracts everything related to an
@@ -502,10 +503,19 @@ static int wait_clear_urbs(struct snd_usb_endpoint *ep)
 	if (alive)
 		snd_printk(KERN_ERR "timeout: still %d active urbs on EP #%x\n",
 					alive, ep->ep_num);
+	clear_bit(EP_FLAG_STOPPING, &ep->flags);
 
 	return 0;
 }
 
+/* wait until urbs are really dropped */
+void snd_usb_endpoint_sync_stop(struct snd_usb_endpoint *ep)
+{
+	if (test_bit(EP_FLAG_STOPPING, &ep->flags))
+		wait_clear_urbs(ep);
+}
+
+
 /*
  * unlink active urbs.
  */
@@ -913,6 +923,8 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep,
 
 		if (wait)
 			wait_clear_urbs(ep);
+		else
+			set_bit(EP_FLAG_STOPPING, &ep->flags);
 	}
 }
 
diff --git a/sound/usb/endpoint.h b/sound/usb/endpoint.h
index cbbbdf2..c1540a4 100644
--- a/sound/usb/endpoint.h
+++ b/sound/usb/endpoint.h
@@ -16,6 +16,7 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep,
 int  snd_usb_endpoint_start(struct snd_usb_endpoint *ep, int can_sleep);
 void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep,
 			   int force, int can_sleep, int wait);
+void snd_usb_endpoint_sync_stop(struct snd_usb_endpoint *ep);
 int  snd_usb_endpoint_activate(struct snd_usb_endpoint *ep);
 int  snd_usb_endpoint_deactivate(struct snd_usb_endpoint *ep);
 void snd_usb_endpoint_free(struct list_head *head);
diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index f782ce1..aee3ab0 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -546,6 +546,11 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
 	if (snd_BUG_ON(!subs->data_endpoint))
 		return -EIO;
 
+	if (subs->sync_endpoint)
+		snd_usb_endpoint_sync_stop(subs->sync_endpoint);
+	if (subs->data_endpoint)
+		snd_usb_endpoint_sync_stop(subs->data_endpoint);
+
 	/* some unit conversions in runtime */
 	subs->data_endpoint->maxframesize =
 		bytes_to_frames(runtime, subs->data_endpoint->maxpacksize);