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

[Takashi Iwai <tiwai@suse.de>]

On Mon, 26 Jun 2023 15:32:40 +0200,
Jaroslav Kysela wrote:
> 
> 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)

I left the optimizations to compilers.  Usually they do inline if it
makes sense, and it's often a more sensible choice.


thanks,

Takashi