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Commit 0d09a9be authored by Glenn Kasten's avatar Glenn Kasten
Browse files

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
parent a2cf13c7

include/private/media/AudioTrackShared.h

+2 −1
Original line number Diff line number Diff line
@@ -94,8 +94,9 @@ struct audio_track_cblk_t 
                                            // 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:

media/libmedia/AudioTrackShared.cpp

+5 −6
Original line number Diff line number Diff line
@@ -207,15 +207,15 @@ status_t ClientProxy::obtainBuffer(Buffer* buffer, const struct timespec *reques 
            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 @@ void ServerProxy::releaseBuffer(Buffer* buffer) 
    }

    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);

        }