iothread: replace fair_mutex with a condition variable
This conveys the intention better, and scales to more than >1 threads contending the mutex with the iothread (as long as all of them have a "quiescent point" like the TCG thread has). Also, on Mac OS X the fair_mutex somehow didn't work as intended and deadlocked. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Tested-by: Alexander Graf <agraf@suse.de> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
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24
cpus.c
24
cpus.c
@ -636,7 +636,8 @@ void vm_stop(int reason)
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#else /* CONFIG_IOTHREAD */
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QemuMutex qemu_global_mutex;
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static QemuMutex qemu_fair_mutex;
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static QemuCond qemu_io_proceeded_cond;
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static bool iothread_requesting_mutex;
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static QemuThread io_thread;
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@ -672,7 +673,7 @@ int qemu_init_main_loop(void)
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qemu_cond_init(&qemu_system_cond);
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qemu_cond_init(&qemu_pause_cond);
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qemu_cond_init(&qemu_work_cond);
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qemu_mutex_init(&qemu_fair_mutex);
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qemu_cond_init(&qemu_io_proceeded_cond);
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qemu_mutex_init(&qemu_global_mutex);
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qemu_mutex_lock(&qemu_global_mutex);
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@ -755,17 +756,9 @@ static void qemu_tcg_wait_io_event(void)
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qemu_cond_wait(tcg_halt_cond, &qemu_global_mutex);
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}
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qemu_mutex_unlock(&qemu_global_mutex);
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/*
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* Users of qemu_global_mutex can be starved, having no chance
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* to acquire it since this path will get to it first.
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* So use another lock to provide fairness.
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*/
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qemu_mutex_lock(&qemu_fair_mutex);
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qemu_mutex_unlock(&qemu_fair_mutex);
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qemu_mutex_lock(&qemu_global_mutex);
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while (iothread_requesting_mutex) {
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qemu_cond_wait(&qemu_io_proceeded_cond, &qemu_global_mutex);
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}
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for (env = first_cpu; env != NULL; env = env->next_cpu) {
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qemu_wait_io_event_common(env);
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@ -908,12 +901,13 @@ void qemu_mutex_lock_iothread(void)
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if (kvm_enabled()) {
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qemu_mutex_lock(&qemu_global_mutex);
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} else {
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qemu_mutex_lock(&qemu_fair_mutex);
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iothread_requesting_mutex = true;
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if (qemu_mutex_trylock(&qemu_global_mutex)) {
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qemu_cpu_kick_thread(first_cpu);
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qemu_mutex_lock(&qemu_global_mutex);
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}
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qemu_mutex_unlock(&qemu_fair_mutex);
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iothread_requesting_mutex = false;
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qemu_cond_broadcast(&qemu_io_proceeded_cond);
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}
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}
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