[PULL 073/115] replay: implement fair mutex

[Paolo Bonzini]

From: Pavel Dovgalyuk <Pavel.Dovgaluk@gmail.com>

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>
---
 replay/replay-internal.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/replay/replay-internal.c b/replay/replay-internal.c
index eba8246aae..2e8a3e947a 100644
--- a/replay/replay-internal.c
+++ b/replay/replay-internal.c
@@ -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);
     }
 }
-- 
2.26.2