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replay: implement fair mutex 

In record/replay icount mode main loop thread and vCPU thread
do not perform simultaneously. They take replay mutex to synchronize
the actions. Sometimes vCPU thread waits for locking the mutex for
very long time, because main loop releases the mutex and takes it
back again. Standard qemu mutex do not provide the ordering
capabilities.

This patch adds a "queue" for replay mutex. Therefore thread ordering
becomes more "fair". Threads are executed in the same order as
they are trying to take the mutex.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
Message-Id: <158823802979.28101.9340462887738957616.stgit@pasha-ThinkPad-X280>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Pavel Dovgalyuk 2020-04-30 12:13:49 +03:00 committed by Paolo Bonzini
parent 2356ff8500
commit ddf63df736
1 changed files with 14 additions and 1 deletions
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15
replay/replay-internal.c
View File

@ -22,6 +22,9 @@
It also protects replay events queue which stores events to be
written or read to the log. */
static QemuMutex lock;
/* Condition and queue for fair ordering of mutex lock requests. */
static QemuCond mutex_cond;
static unsigned long mutex_head, mutex_tail;
/* File for replay writing */
static bool write_error;
@ -197,9 +200,10 @@ static __thread bool replay_locked;
void replay_mutex_init(void)
{
qemu_mutex_init(&lock);
qemu_cond_init(&mutex_cond);
/* Hold the mutex while we start-up */
qemu_mutex_lock(&lock);
replay_locked = true;
++mutex_tail;
}
bool replay_mutex_locked(void)
@ -211,10 +215,16 @@ bool replay_mutex_locked(void)
void replay_mutex_lock(void)
{
if (replay_mode != REPLAY_MODE_NONE) {
unsigned long id;
g_assert(!qemu_mutex_iothread_locked());
g_assert(!replay_mutex_locked());
qemu_mutex_lock(&lock);
id = mutex_tail++;
while (id != mutex_head) {
qemu_cond_wait(&mutex_cond, &lock);
}
replay_locked = true;
qemu_mutex_unlock(&lock);
}
}
@ -222,7 +232,10 @@ void replay_mutex_unlock(void)
{
if (replay_mode != REPLAY_MODE_NONE) {
g_assert(replay_mutex_locked());
qemu_mutex_lock(&lock);
++mutex_head;
replay_locked = false;
qemu_cond_broadcast(&mutex_cond);
qemu_mutex_unlock(&lock);
}
}