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Date:   Mon, 26 Jun 2023 15:32:40 +0200
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To:     Takashi Iwai <tiwai@suse.de>
Cc:     Tuo Li <islituo@gmail.com>, tiwai@suse.com,
        alsa-devel@alsa-project.org,
        Linux Kernel <linux-kernel@vger.kernel.org>,
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From:   Jaroslav Kysela <perex@perex.cz>
Subject: Re: [BUG] ALSA: core: pcm_memory: a possible data race in
 do_alloc_pages()
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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@perex.cz>

				Thanks,
					Jaroslav

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