Re: [BUG] ALSA: core: pcm_memory: a possible data race in do_alloc_pages()

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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@xxxxxxxx>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.




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