Use mFutex as an event flag rather than semaphore
An event flag can be more fault-tolerant in case of loss of synchronization,
as it cannot overflow. It also allows more bits to be used in the future.
See http://en.wikipedia.org/wiki/Event_flag
Change-Id: I01ca25d951eb263124da54bb4738f0d94ec4a48b
diff --git a/include/private/media/AudioTrackShared.h b/include/private/media/AudioTrackShared.h
index 681f557..ef5bb8d 100644
--- a/include/private/media/AudioTrackShared.h
+++ b/include/private/media/AudioTrackShared.h
@@ -94,8 +94,9 @@
// parameter
// renamed to "_" to detect incorrect use
- volatile int32_t mFutex; // semaphore: down (P) by client,
+ volatile int32_t mFutex; // event flag: down (P) by client,
// up (V) by server or binderDied() or interrupt()
+#define CBLK_FUTEX_WAKE 1 // if event flag bit is set, then a deferred wake is pending
private:
diff --git a/media/libmedia/AudioTrackShared.cpp b/media/libmedia/AudioTrackShared.cpp
index f034164..4b7f368 100644
--- a/media/libmedia/AudioTrackShared.cpp
+++ b/media/libmedia/AudioTrackShared.cpp
@@ -207,15 +207,15 @@
ts = NULL;
break;
}
- int32_t old = android_atomic_dec(&cblk->mFutex);
- if (old <= 0) {
+ int32_t old = android_atomic_and(~CBLK_FUTEX_WAKE, &cblk->mFutex);
+ if (!(old & CBLK_FUTEX_WAKE)) {
int rc;
if (measure && !beforeIsValid) {
clock_gettime(CLOCK_MONOTONIC, &before);
beforeIsValid = true;
}
int ret = __futex_syscall4(&cblk->mFutex,
- mClientInServer ? FUTEX_WAIT_PRIVATE : FUTEX_WAIT, old - 1, ts);
+ mClientInServer ? FUTEX_WAIT_PRIVATE : FUTEX_WAIT, old & ~CBLK_FUTEX_WAKE, ts);
// update total elapsed time spent waiting
if (measure) {
struct timespec after;
@@ -484,9 +484,8 @@
}
if (!mDeferWake && mAvailToClient + stepCount >= minimum) {
ALOGV("mAvailToClient=%u stepCount=%u minimum=%u", mAvailToClient, stepCount, minimum);
- // could client be sleeping, or not need this increment and counter overflows?
- int32_t old = android_atomic_inc(&cblk->mFutex);
- if (old == -1) {
+ int32_t old = android_atomic_or(CBLK_FUTEX_WAKE, &cblk->mFutex);
+ if (!(old & CBLK_FUTEX_WAKE)) {
(void) __futex_syscall3(&cblk->mFutex,
mClientInServer ? FUTEX_WAKE_PRIVATE : FUTEX_WAKE, 1);
}