linux-kernel - Re: snd_card_disconnect race (sound/core/init.c)

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Date:	Thu, 24 Mar 2011 01:19:43 -0700
From:	Russ Dill <russ.dill@...il.com>
To:	Takashi Iwai <tiwai@...e.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: snd_card_disconnect race (sound/core/init.c)

On Wed, Mar 23, 2011 at 11:43 PM, Takashi Iwai <tiwai@...e.de> wrote:
> At Wed, 23 Mar 2011 18:40:58 -0700,
> Russ Dill wrote:
>>
>> With slub poisoning on, I'm seeing an oops in snd_disconnect_release
>> with slub poison (6b6b6b6b). Investigating, snd_card_disconnect looks
>> to be the possible culprit:
>>
>> 333         spin_lock(&card->files_lock);
>> 334         mfile = card->files;
>> 335         while (mfile) {
>> 336                 file = mfile->file;
>> 337
>> 338                 /* it's critical part, use endless loop */
>> 339                 /* we have no room to fail */
>> 340                 mfile->disconnected_f_op = mfile->file->f_op;
>> 341
>> 342                 spin_lock(&shutdown_lock);
>> 343                 list_add(&mfile->shutdown_list, &shutdown_files);
>> 344                 spin_unlock(&shutdown_lock);
>> 345
>> 346                 mfile->file->f_op = &snd_shutdown_f_ops;
>> 347                 fops_get(mfile->file->f_op);
>> 348
>> 349                 mfile = mfile->next;
>> 350         }
>> 351         spin_unlock(&card->files_lock);
>>
>> I'm not aware of any locking that would prevent the original release
>> function running concurrently if it were started before line 346 was
>> executed. The original release functions (such as snd_hwdep_release)
>> call snd_card_remove_file which frees the mfile object which would
>> have been just added to the shutdown_files list leading to a use of
>> freed (or in my case poisoned) memory later on down the line when the
>> shutdown_files gets walked again.
>>
>> It seems that 118dd6bf (ALSA: Clean up snd_monitor_file management,
>> v2.6.30) may have either introduced this problem or made it worse
>> since snd_card_file_remove previously removed items from the
>> shutdown_files list before freeing them.
>>
>> I'm currently experiencing these crashes on 2.6.32.26, but I can't
>> find any changes that would cause them not to occur on later kernel
>> versions.
>
> Which device are you using?  USB-audio had a known problem at
> disconnection, for example, and it was fixed recently.
>

I'm using USB-audio. I tried back-porting a couple of patches, to no avail:

ALSA: usb-audio: fix oops due to cleanup race when disconnecting
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=382225e6

ALSA: usb - Release capture substream URBs properly
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=76195fb0

Backing out 118dd6bf fixes the problem. The app that is running when
this crash occurs does full duplex streaming and segfaults while the
USB-audio device is going away. I'd have to check, but I think its
actually segfaulting it response to a disconnect of another piece of
the composite device the USB-audio device is part of, so the release
may actually be happening at the time the snd_card_disconnect is
occurring. I don't see any locking that would prevent bad things from
happening in this case, and the shutdown_files list is definitely
poisoned.
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