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

[Takashi Iwai <tiwai@suse.de>]

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)
+{
+	card->total_pcm_alloc_bytes += bytes;
+}
+
+static void update_allocated_size(struct snd_card *card, ssize_t bytes)
+{
+	mutex_lock(&card->memory_mutex);
+	__update_allocated_size(card, bytes);
+	mutex_unlock(&card->memory_mutex);
+}
+
+static void decrease_allocated_size(struct snd_card *card, size_t bytes)
+{
+	mutex_lock(&card->memory_mutex);
+	WARN_ON(card->total_pcm_alloc_bytes < bytes);
+	__update_allocated_size(card, -(ssize_t)bytes);
+	mutex_unlock(&card->memory_mutex);
+}
+
 static int do_alloc_pages(struct snd_card *card, int type, struct device *dev,
 			  int str, size_t size, struct snd_dma_buffer *dmab)
 {
 	enum dma_data_direction dir;
 	int err;
 
+	/* check and reserve the requested size */
+	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;
+	}
+	__update_allocated_size(card, size);
+	mutex_unlock(&card->memory_mutex);
 
 	if (str == SNDRV_PCM_STREAM_PLAYBACK)
 		dir = DMA_TO_DEVICE;
@@ -47,9 +73,14 @@ 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) {
-		mutex_lock(&card->memory_mutex);
-		card->total_pcm_alloc_bytes += dmab->bytes;
-		mutex_unlock(&card->memory_mutex);
+		/* the actual allocation size might be bigger than requested,
+		 * and we need to correct the account
+		 */
+		if (dmab->bytes != size)
+			update_allocated_size(card, dmab->bytes - size);
+	} else {
+		/* take back on allocation failure */
+		decrease_allocated_size(card, size);
 	}
 	return err;
 }
@@ -58,10 +89,7 @@ static void do_free_pages(struct snd_card *card, struct snd_dma_buffer *dmab)
 {
 	if (!dmab->area)
 		return;
-	mutex_lock(&card->memory_mutex);
-	WARN_ON(card->total_pcm_alloc_bytes < dmab->bytes);
-	card->total_pcm_alloc_bytes -= dmab->bytes;
-	mutex_unlock(&card->memory_mutex);
+	decrease_allocated_size(card, dmab->bytes);
 	snd_dma_free_pages(dmab);
 	dmab->area = NULL;
 }