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 <pbonz...@redhat.com> Tested-by: Alexander Graf <ag...@suse.de> --- cpus.c | 24 +++++++++--------------- 1 files changed, 9 insertions(+), 15 deletions(-) diff --git a/cpus.c b/cpus.c index 1fc34b7..d2ba933 100644 --- a/cpus.c +++ b/cpus.c @@ -626,7 +626,8 @@ void vm_stop(int reason) #else /* CONFIG_IOTHREAD */ QemuMutex qemu_global_mutex; -static QemuMutex qemu_fair_mutex; +static QemuCond qemu_io_proceeded_cond; +static bool iothread_requesting_mutex; static QemuThread io_thread; @@ -662,7 +663,7 @@ int qemu_init_main_loop(void) qemu_cond_init(&qemu_system_cond); qemu_cond_init(&qemu_pause_cond); qemu_cond_init(&qemu_work_cond); - qemu_mutex_init(&qemu_fair_mutex); + qemu_cond_init(&qemu_io_proceeded_cond); qemu_mutex_init(&qemu_global_mutex); qemu_mutex_lock(&qemu_global_mutex); @@ -745,17 +746,9 @@ static void qemu_tcg_wait_io_event(void) qemu_cond_wait(tcg_halt_cond, &qemu_global_mutex); } - qemu_mutex_unlock(&qemu_global_mutex); - - /* - * Users of qemu_global_mutex can be starved, having no chance - * to acquire it since this path will get to it first. - * So use another lock to provide fairness. - */ - qemu_mutex_lock(&qemu_fair_mutex); - qemu_mutex_unlock(&qemu_fair_mutex); - - qemu_mutex_lock(&qemu_global_mutex); + while (iothread_requesting_mutex) { + qemu_cond_wait(&qemu_io_proceeded_cond, &qemu_global_mutex); + } for (env = first_cpu; env != NULL; env = env->next_cpu) { qemu_wait_io_event_common(env); @@ -898,12 +891,13 @@ void qemu_mutex_lock_iothread(void) if (kvm_enabled()) { qemu_mutex_lock(&qemu_global_mutex); } else { - qemu_mutex_lock(&qemu_fair_mutex); + iothread_requesting_mutex = true; if (qemu_mutex_trylock(&qemu_global_mutex)) { qemu_cpu_kick_thread(first_cpu); qemu_mutex_lock(&qemu_global_mutex); } - qemu_mutex_unlock(&qemu_fair_mutex); + iothread_requesting_mutex = false; + qemu_cond_broadcast(&qemu_io_proceeded_cond); } } -- 1.7.4.4