Commit 67ec1072b053 ("ALSA: pcm: Fix rwsem deadlock for non-atomic PCM stream")
fixes deadlock for non-atomic PCM stream. But, This patch causes antother stuck.
If writer is RT thread and reader is a normal thread, the reader thread will
be difficult to get scheduled. It may not give chance to release readlocks
and writer gets stuck for a long time if they are pinned to single cpu.
The deadlock described in the previous commit is because the linux rwsem
queues like a FIFO. So, we might need non-FIFO writelock, not non-block one.
My suggestion is that the writer gives reader a chance to be scheduled by using
the minimum msleep() instaed of spinning without blocking by writer. Also,
The *_nonblock may be changed to *_nonfifo appropriately to this concept.
In terms of performance, when trylock is failed, this minimum periodic msleep
will have the same performance as the tick-based schedule()/wake_up_q().
Suggested-by: Wonmin Jung <wonmin.jung@xxxxxxx>
Signed-off-by: Chanho Min <chanho.min@xxxxxxx>
---
sound/core/pcm_native.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 66c90f4..bdca0e1 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -36,6 +36,7 @@
#include <sound/timer.h>
#include <sound/minors.h>
#include <linux/uio.h>
+#include <linux/delay.h>
#include "pcm_local.h"
@@ -91,12 +92,12 @@ static DECLARE_RWSEM(snd_pcm_link_rwsem);
* and this may lead to a deadlock when the code path takes read sem
* twice (e.g. one in snd_pcm_action_nonatomic() and another in
* snd_pcm_stream_lock()). As a (suboptimal) workaround, let writer to
- * spin until it gets the lock.
+ * sleep until all the readers are completed without blocking by writer.
*/
-static inline void down_write_nonblock(struct rw_semaphore *lock)
+static inline void down_write_nonfifo(struct rw_semaphore *lock)
{
while (!down_write_trylock(lock))
- cond_resched();
+ msleep(1);
}
#define PCM_LOCK_DEFAULT 0
@@ -1967,7 +1968,7 @@ static int snd_pcm_link(struct snd_pcm_substream *substream, int fd)
res = -ENOMEM;
goto _nolock;
}
- down_write_nonblock(&snd_pcm_link_rwsem);
+ down_write_nonfifo(&snd_pcm_link_rwsem);
write_lock_irq(&snd_pcm_link_rwlock);
if (substream->runtime->status->state == SNDRV_PCM_STATE_OPEN ||
substream->runtime->status->state != substream1->runtime->status->state ||
@@ -2014,7 +2015,7 @@ static int snd_pcm_unlink(struct snd_pcm_substream *substream)
struct snd_pcm_substream *s;
int res = 0;
- down_write_nonblock(&snd_pcm_link_rwsem);
+ down_write_nonfifo(&snd_pcm_link_rwsem);
write_lock_irq(&snd_pcm_link_rwlock);
if (!snd_pcm_stream_linked(substream)) {
res = -EALREADY;
--
2.1.4
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