File 0027-pcm-direct-fix-race-on-clearing-timer-even... of Package alsa

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File 0027-pcm-direct-fix-race-on-clearing-timer-events.patch of Package alsa (Revision 718f2436661872f77df52b9e10df1977)

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From 9219034301a3bf3d8de57ac5672bbc1c53f18049 Mon Sep 17 00:00:00 2001
From: Andreas Pape <apape@de.adit-jv.com>
Date: Fri, 17 Feb 2017 12:45:56 +0530
Subject: [PATCH 27/43] pcm: direct: fix race on clearing timer events

snd_timer handling is racy: plugins clear timer queue if avail_min
is not reached to force a sleep on timer. The race can happen if
the expected event arrives in between the avail check and the
clearing of pending events. If this race happens, the user will
unnecessarily wait for one more timer event. On low latency/realtime
streams this can lead to xruns and must be avoided.

As a fix we recheck avail after having cleared poll events.

Signed-off-by: Andreas Pape <apape@de.adit-jv.com>
Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Mounesh Sutar <sutar.mounesh@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 src/pcm/pcm_direct.c |   23 +++++++++++++++++++----
 src/pcm/pcm_direct.h |    2 +-
 2 files changed, 20 insertions(+), 5 deletions(-)

--- a/src/pcm/pcm_direct.c
+++ b/src/pcm/pcm_direct.c
@@ -515,10 +515,12 @@ int snd_pcm_direct_async(snd_pcm_t *pcm,
 }
 
 /* empty the timer read queue */
-void snd_pcm_direct_clear_timer_queue(snd_pcm_direct_t *dmix)
+int snd_pcm_direct_clear_timer_queue(snd_pcm_direct_t *dmix)
 {
+	int changed = 0;
 	if (dmix->timer_need_poll) {
 		while (poll(&dmix->timer_fd, 1, 0) > 0) {
+			changed++;
 			/* we don't need the value */
 			if (dmix->tread) {
 				snd_timer_tread_t rbuf[4];
@@ -533,15 +535,17 @@ void snd_pcm_direct_clear_timer_queue(sn
 			snd_timer_tread_t rbuf[4];
 			int len;
 			while ((len = snd_timer_read(dmix->timer, rbuf,
-						     sizeof(rbuf))) > 0 &&
+						     sizeof(rbuf))) > 0
+						     && (++changed) &&
 			       len != sizeof(rbuf[0]))
 				;
 		} else {
 			snd_timer_read_t rbuf;
 			while (snd_timer_read(dmix->timer, &rbuf, sizeof(rbuf)) > 0)
-				;
+				changed++;
 		}
 	}
+	return changed;
 }
 
 int snd_pcm_direct_timer_stop(snd_pcm_direct_t *dmix)
@@ -693,6 +697,8 @@ int snd_pcm_direct_poll_revents(snd_pcm_
 	int empty = 0;
 
 	assert(pfds && nfds == 1 && revents);
+
+timer_changed:
 	events = pfds[0].revents;
 	if (events & POLLIN) {
 		snd_pcm_uframes_t avail;
@@ -720,7 +726,16 @@ int snd_pcm_direct_poll_revents(snd_pcm_
 		break;
 	default:
 		if (empty) {
-			snd_pcm_direct_clear_timer_queue(dmix);
+			/* here we have a race condition:
+			 * if period event arrived after the avail_update call
+			 * above we might clear this event with the following
+			 * clear_timer_queue.
+			 * There is no way to do this in atomic manner, so we
+			 * need to recheck avail_update if we successfully
+			 * cleared a poll event.
+			 */
+			if (snd_pcm_direct_clear_timer_queue(dmix))
+				goto timer_changed;
 			events &= ~(POLLOUT|POLLIN);
 			/* additional check */
 			switch (__snd_pcm_state(pcm)) {
--- a/src/pcm/pcm_direct.h
+++ b/src/pcm/pcm_direct.h
@@ -322,7 +322,7 @@ int snd_pcm_direct_munmap(snd_pcm_t *pcm
 int snd_pcm_direct_prepare(snd_pcm_t *pcm);
 int snd_pcm_direct_resume(snd_pcm_t *pcm);
 int snd_pcm_direct_timer_stop(snd_pcm_direct_t *dmix);
-void snd_pcm_direct_clear_timer_queue(snd_pcm_direct_t *dmix);
+int snd_pcm_direct_clear_timer_queue(snd_pcm_direct_t *dmix);
 int snd_pcm_direct_set_timer_params(snd_pcm_direct_t *dmix);
 int snd_pcm_direct_open_secondary_client(snd_pcm_t **spcmp, snd_pcm_direct_t *dmix, const char *client_name);

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File 0027-pcm-direct-fix-race-on-clearing-timer-events.patch of Package alsa (Revision 718f2436661872f77df52b9e10df1977)

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