[FFmpeg-cvslog] pthread: Avoid spurious wakeups

[Ben Jackson]

ffmpeg | branch: release/1.1 | Ben Jackson <ben at ben.com> | Fri Oct 18 15:28:50 2013 +0100| [311583e7798237be5cc531d672a9e37f8c729d83] | committer: Sean McGovern

pthread: Avoid spurious wakeups

pthread_wait_cond can wake up unexpectedly (Wikipedia: Spurious_wakeup).

The FF_THREAD_SLICE thread mechanism could spontaneously execute
jobs or allow the caller of avctx->execute to return before all
jobs were complete.

Test both cases to ensure the wakeup is real.

Signed-off-by: Ben Jackson <ben at ben.com>
Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis at gmail.com>
Signed-off-by: Luca Barbato <lu_zero at gentoo.org>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=311583e7798237be5cc531d672a9e37f8c729d83
---

 libavcodec/pthread.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/libavcodec/pthread.c b/libavcodec/pthread.c
index 5ebcc9b..8ae494b 100644
--- a/libavcodec/pthread.c
+++ b/libavcodec/pthread.c
@@ -76,6 +76,7 @@ typedef struct ThreadContext {
     pthread_cond_t last_job_cond;
     pthread_cond_t current_job_cond;
     pthread_mutex_t current_job_lock;
+    unsigned current_execute;
     int current_job;
     int done;
 } ThreadContext;
@@ -196,6 +197,7 @@ static void* attribute_align_arg worker(void *v)
 {
     AVCodecContext *avctx = v;
     ThreadContext *c = avctx->thread_opaque;
+    unsigned last_execute = 0;
     int our_job = c->job_count;
     int thread_count = avctx->thread_count;
     int self_id;
@@ -207,8 +209,9 @@ static void* attribute_align_arg worker(void *v)
             if (c->current_job == thread_count + c->job_count)
                 pthread_cond_signal(&c->last_job_cond);
 
-            if (!c->done)
+            while (last_execute == c->current_execute && !c->done)
                 pthread_cond_wait(&c->current_job_cond, &c->current_job_lock);
+            last_execute = c->current_execute;
             our_job = self_id;
 
             if (c->done) {
@@ -228,7 +231,8 @@ static void* attribute_align_arg worker(void *v)
 
 static av_always_inline void avcodec_thread_park_workers(ThreadContext *c, int thread_count)
 {
-    pthread_cond_wait(&c->last_job_cond, &c->current_job_lock);
+    while (c->current_job != thread_count + c->job_count)
+        pthread_cond_wait(&c->last_job_cond, &c->current_job_lock);
     pthread_mutex_unlock(&c->current_job_lock);
 }
 
@@ -277,6 +281,7 @@ static int avcodec_thread_execute(AVCodecContext *avctx, action_func* func, void
         c->rets = &dummy_ret;
         c->rets_count = 1;
     }
+    c->current_execute++;
     pthread_cond_broadcast(&c->current_job_cond);
 
     avcodec_thread_park_workers(c, avctx->thread_count);