Re: [PATCH] ALSA: core: Replace mutex_lock with mutex_trylock

[Takashi Iwai <tiwai@suse.de>]

On Wed, 08 Nov 2023 13:39:22 +0100,
Takashi Iwai wrote:
> 
> On Wed, 08 Nov 2023 13:14:15 +0100,
> 강신형 wrote:
> > 
> > > Thanks for the patch.  But this change may break the current working
> > > behavior; e.g. when two proc reads are running concurrently, one would
> > > be aborted unexpectedly.
> > > 
> > > IIUC, the problem is the call of proc_remove(), and this call itself
> > > can be outside the global mutex.
> > > 
> > > Could you check whether the patch below works instead?  (Note that
> > > it's only compile-tested.)  It makes the proc_remove() called at
> > > first, then clearing the internal entries.  The function was renamed
> > > accordingly for avoiding confusion, too.
> > > 
> > > 
> > > Takashi
> > 
> > You are right. My patch is just for avoiding the deadlock. 
> > It may lead to other problem instead the deadlock(e.g. USB sound card
> > registration failure)
> > Your patch works well without any problems.
> > But I can't confirm that the problem is solved or not.
> > because the issue has occurred only once until now.
> > (Test method: USB insertion / removal during a call)
> 
> Maybe you can reproduce it more easily by adding some delay
> (e.g. ssleep(2)) before mutex_lock() in snd_info_entry_open().

... in our case, it's rather snd_info_text_entry_open().

> Then it's easier to cause a race.

I could actually reproduce the bug with the extra delay, and confirmed
that my patch works around it.
I'm going to submit a proper patch.


Takashi