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

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Message-ID: <45445f57-0a73-59e6-6f3d-3983ce93a324@perex.cz>
Date:   Mon, 26 Jun 2023 13:09:00 +0200
From:   Jaroslav Kysela <perex@...ex.cz>
To:     Takashi Iwai <tiwai@...e.de>
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 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.

					Jaroslav

-- 
Jaroslav Kysela <perex@...ex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.