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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>
This commit is contained in:
Paolo Bonzini 2011-06-09 13:10:24 +02:00 committed by Anthony Liguori
parent 49e40b6627
commit 46daff13c8
1 changed files with 9 additions and 15 deletions
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24
cpus.c
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@ -636,7 +636,8 @@ void vm_stop(int reason)
#else /* CONFIG_IOTHREAD */ #else /* CONFIG_IOTHREAD */
QemuMutex qemu_global_mutex; QemuMutex qemu_global_mutex;
static QemuMutex qemu_fair_mutex; static QemuCond qemu_io_proceeded_cond;
static bool iothread_requesting_mutex;
static QemuThread io_thread; static QemuThread io_thread;
@ -672,7 +673,7 @@ int qemu_init_main_loop(void)
qemu_cond_init(&qemu_system_cond); qemu_cond_init(&qemu_system_cond);
qemu_cond_init(&qemu_pause_cond); qemu_cond_init(&qemu_pause_cond);
qemu_cond_init(&qemu_work_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_init(&qemu_global_mutex);
qemu_mutex_lock(&qemu_global_mutex); qemu_mutex_lock(&qemu_global_mutex);
@ -755,17 +756,9 @@ static void qemu_tcg_wait_io_event(void)
qemu_cond_wait(tcg_halt_cond, &qemu_global_mutex); qemu_cond_wait(tcg_halt_cond, &qemu_global_mutex);
} }
qemu_mutex_unlock(&qemu_global_mutex); while (iothread_requesting_mutex) {
qemu_cond_wait(&qemu_io_proceeded_cond, &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);
for (env = first_cpu; env != NULL; env = env->next_cpu) { for (env = first_cpu; env != NULL; env = env->next_cpu) {
qemu_wait_io_event_common(env); qemu_wait_io_event_common(env);
@ -908,12 +901,13 @@ void qemu_mutex_lock_iothread(void)
if (kvm_enabled()) { if (kvm_enabled()) {
qemu_mutex_lock(&qemu_global_mutex); qemu_mutex_lock(&qemu_global_mutex);
} else { } else {
qemu_mutex_lock(&qemu_fair_mutex); iothread_requesting_mutex = true;
if (qemu_mutex_trylock(&qemu_global_mutex)) { if (qemu_mutex_trylock(&qemu_global_mutex)) {
qemu_cpu_kick_thread(first_cpu); qemu_cpu_kick_thread(first_cpu);
qemu_mutex_lock(&qemu_global_mutex); qemu_mutex_lock(&qemu_global_mutex);
} }
qemu_mutex_unlock(&qemu_fair_mutex); iothread_requesting_mutex = false;
qemu_cond_broadcast(&qemu_io_proceeded_cond);
} }
} }