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

[Jaroslav Kysela]

On 26. 06. 23 15:15, Takashi Iwai wrote:
> On Mon, 26 Jun 2023 13:13:21 +0200,
> Takashi Iwai wrote:
> > 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.
>
> ... and here it is.
>
> If that looks OK, I'll submit a proper fix patch.
>
>
> thanks,
>
> Takashi
>
> --- a/sound/core/pcm_memory.c
> +++ b/sound/core/pcm_memory.c
> @@ -31,15 +31,41 @@ static unsigned long max_alloc_per_card = 32UL * 1024UL * 1024UL;
>   module_param(max_alloc_per_card, ulong, 0644);
>   MODULE_PARM_DESC(max_alloc_per_card, "Max total allocation bytes per card.");
>   
> +static void __update_allocated_size(struct snd_card *card, ssize_t bytes)

Missing inline ? May be also used for

> +static void update_allocated_size(struct snd_card *card, ssize_t bytes)
> +static void decrease_allocated_size(struct snd_card *card, size_t bytes)

The rest is fine in my eyes.

Reviewed-by: Jaroslav Kysela <perex@xxxxxxxx>

				Thanks,
					Jaroslav

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