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[Qemu-devel] [PATCH 1/2] iothread: replace fair_mutex with a condition v
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [PATCH 1/2] iothread: replace fair_mutex with a condition variable |
Date: |
Thu, 9 Jun 2011 13:10:24 +0200 |
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 <address@hidden>
Tested-by: Alexander Graf <address@hidden>
---
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