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

[Takashi Iwai]

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.

> Eventually, total_pcm_alloc_bytes may be atomic.

Possible, but the same problem like the above applies, so I believe
the mutex is good enough.

Another alternative would be to move the size check after the
successful allocation, assuming that the size exceeds a very
exceptional scenario.  The code flow would be a bit simpler.


thanks,

Takashi

--- a/sound/core/pcm_memory.c
+++ b/sound/core/pcm_memory.c
@@ -37,9 +37,14 @@ static int do_alloc_pages(struct snd_card *card, int type, struct device *dev,
 	enum dma_data_direction dir;
 	int err;
 
+	mutex_lock(&card->memory_mutex);
 	if (max_alloc_per_card &&
-	    card->total_pcm_alloc_bytes + size > max_alloc_per_card)
+	    card->total_pcm_alloc_bytes + size > max_alloc_per_card) {
+		mutex_unlock(&card->memory_mutex);
 		return -ENOMEM;
+	}
+	card->total_pcm_alloc_bytes += size		
+	mutex_unlock(&card->memory_mutex);
 
 	if (str == SNDRV_PCM_STREAM_PLAYBACK)
 		dir = DMA_TO_DEVICE;
@@ -47,8 +52,18 @@ static int do_alloc_pages(struct snd_card *card, int type, struct device *dev,
 		dir = DMA_FROM_DEVICE;
 	err = snd_dma_alloc_dir_pages(type, dev, dir, size, dmab);
 	if (!err) {
+		/* the actual allocation size might be bigger than requested,
+		 * and we need to correct the account
+		 */
+		if (dmab->bytes != size) {
+			mutex_lock(&card->memory_mutex);
+			card->total_pcm_alloc_bytes += dmab->bytes - size;
+			mutex_unlock(&card->memory_mutex);
+		}
+	} else {
+		/* allocation failure, take back */
 		mutex_lock(&card->memory_mutex);
-		card->total_pcm_alloc_bytes += dmab->bytes;
+		card->total_pcm_alloc_bytes -= size;
 		mutex_unlock(&card->memory_mutex);
 	}
 	return err;