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

[Takashi Iwai <tiwai@suse.de>]

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.


Takashi