linux-kernel - Re: [BUG] ALSA: core: pcm_memory: a possible data race in do_alloc

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Message-ID: <87ttuuv5m6.wl-tiwai@suse.de>
Date:   Mon, 26 Jun 2023 13:13:21 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Jaroslav Kysela <perex@...ex.cz>
Cc:     Tuo Li <islituo@...il.com>, tiwai@...e.com,
        alsa-devel@...a-project.org,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        baijiaju1990@...look.com
Subject: Re: [BUG] ALSA: core: pcm_memory: a possible data race in do_alloc_pages()

On Mon, 26 Jun 2023 13:09:00 +0200,
Jaroslav Kysela wrote:
> 
> On 26. 06. 23 13:02, Takashi Iwai wrote:
> > On Mon, 26 Jun 2023 09:56:47 +0200,
> > Jaroslav Kysela wrote:
> >> 
> >> On 26. 06. 23 9:33, Takashi Iwai wrote:
> >>> On Mon, 26 Jun 2023 09:31:18 +0200,
> >>> Tuo Li wrote:
> >>>> 
> >>>> 
> >>>> Hello,
> >>>> 
> >>>> Thank you for your reply!
> >>> 
> >>> FWIW, the simplest fix would be something like below, just extending
> >>> the mutex coverage.  But it'll serialize the all calls, so it might
> >>> influence on the performance, while it's the safest way.
> >> 
> >> It may be better to update total_pcm_alloc_bytes before
> >> snd_dma_alloc_dir_pages() call and decrease this value when allocation
> >> fails to allow parallel allocations. Then the mutex can be held only
> >> for the total_pcm_alloc_bytes variable update.
> > 
> > Yes, it'd work.  But a tricky part is that the actual allocation size
> > can be bigger, and we need to correct the total_pcm_alloc_bytes after
> > the allocation result.  So the end result would be a patch like below,
> > which is a bit more complex than the previous simpler approach.  But
> > it might be OK.
> 
> The patch looks good, but it may be better to move the "post" variable
> updates to an inline function (mutex lock - update - mutex unlock) for
> a better readability.

Sounds like a good idea.  Let me cook later.


Takashi