Re: [Qemu-devel] [PATCH v2] trace: add qemu mutex lock and unlock trace events

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/04/2017 19:19, Jose Ricardo Ziviani wrote:
> These trace events were very useful to help me to understand and find a
> reordering issue in vfio, for example:
> 
> qemu_mutex_lock locked mutex 0x10905ad8
>   vfio_region_write  (0001:03:00.0:region1+0xc0, 0x2020c, 4)
> qemu_mutex_unlock unlocked mutex 0x10905ad8
> qemu_mutex_lock locked mutex 0x10905ad8
>   vfio_region_write  (0001:03:00.0:region1+0xc4, 0xa0000, 4)
> qemu_mutex_unlock unlocked mutex 0x10905ad8
> 
> that also helped me to see the desired result after the fix:
> 
> qemu_mutex_lock locked mutex 0x10905ad8
>   vfio_region_write  (0001:03:00.0:region1+0xc0, 0x2000c, 4)
>   vfio_region_write  (0001:03:00.0:region1+0xc4, 0xb0000, 4)
> qemu_mutex_unlock unlocked mutex 0x10905ad8
> 
> So it could be a good idea to have these traces implemented. It's worth
> mentioning that they should be surgically enabled during the debugging,
> otherwise it can flood the trace logs with lock/unlock messages.
> 
> How to use it:
> trace-event qemu_mutex_lock on|off
> trace-event qemu_mutex_unlock on|off
> or
> trace-event qemu_mutex* on|off
> 
> Signed-off-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>

Some improvements:

1) handle trylock and Win32 too

2) pass mutex instead of &mutex->lock, it is the same but the latter is
unnecessarily obfuscated

3) also trace unlock/lock around cond_wait

4) trace "unlocked" before calling pthread_mutex_unlock, so that it is
always placed before the next "locked" tracepoint.

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index bf5756763d..46f4c08e6d 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -62,7 +62,7 @@ void qemu_mutex_lock(QemuMutex *mutex)
     if (err)
         error_exit(err, __func__);

-    trace_qemu_mutex_lock(&mutex->lock);
+    trace_qemu_mutex_locked(mutex);
 }

 int qemu_mutex_trylock(QemuMutex *mutex)
@@ -71,7 +71,7 @@ int qemu_mutex_trylock(QemuMutex *mutex)

     err = pthread_mutex_trylock(&mutex->lock);
     if (err == 0) {
-        trace_qemu_mutex_lock(&mutex->lock);
+        trace_qemu_mutex_locked(mutex);
         return 0;
     }
     if (err == EBUSY) {
@@ -84,11 +84,10 @@ void qemu_mutex_unlock(QemuMutex *mutex)
 {
     int err;

+    trace_qemu_mutex_unlocked(mutex);
     err = pthread_mutex_unlock(&mutex->lock);
     if (err)
         error_exit(err, __func__);
-
-    trace_qemu_mutex_unlock(&mutex->lock);
 }

 void qemu_rec_mutex_init(QemuRecMutex *mutex)
@@ -145,7 +144,9 @@ void qemu_cond_wait(QemuCond *cond, QemuMutex *mutex)
 {
     int err;

+    trace_qemu_mutex_unlocked(mutex);
     err = pthread_cond_wait(&cond->cond, &mutex->lock);
+    trace_qemu_mutex_locked(mutex);
     if (err)
         error_exit(err, __func__);
 }
diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
index d3c87bc89e..0dc3ae7756 100644
--- a/util/qemu-thread-win32.c
+++ b/util/qemu-thread-win32.c
@@ -55,6 +55,7 @@ void qemu_mutex_destroy(QemuMutex *mutex)
 void qemu_mutex_lock(QemuMutex *mutex)
 {
     AcquireSRWLockExclusive(&mutex->lock);
+    trace_qemu_mutex_locked(mutex);
 }

 int qemu_mutex_trylock(QemuMutex *mutex)
@@ -64,6 +64,7 @@ int qemu_mutex_trylock(QemuMutex *mutex)

     owned = TryAcquireSRWLockExclusive(&mutex->lock);
     if (owned) {
+        trace_qemu_mutex_locked(mutex);
         return 0;
     }
     return -EBUSY;
@@ -72,6 +72,7 @@ int qemu_mutex_trylock(QemuMutex *mutex)

 void qemu_mutex_unlock(QemuMutex *mutex)
 {
+    trace_qemu_mutex_unlocked(mutex);
     ReleaseSRWLockExclusive(&mutex->lock);
 }

@@ -124,7 +124,9 @@ void qemu_cond_broadcast(QemuCond *cond)

 void qemu_cond_wait(QemuCond *cond, QemuMutex *mutex)
 {
+    trace_qemu_mutex_unlocked(mutex);
     SleepConditionVariableSRW(&cond->var, &mutex->lock, INFINITE, 0);
+    trace_qemu_mutex_locked(mutex);
 }

 void qemu_sem_init(QemuSemaphore *sem, int init)
diff --git a/util/trace-events b/util/trace-events
index 70f62124e1..fa540c620b 100644
--- a/util/trace-events
+++ b/util/trace-events
@@ -57,5 +57,5 @@ lockcnt_futex_wait_resume(const void *lockcnt, int
new) "lockcnt %p after wait:
 lockcnt_futex_wake(const void *lockcnt) "lockcnt %p waking up one waiter"

 # util/qemu-thread-posix.c
-qemu_mutex_lock(void *lock) "locked mutex %p"
-qemu_mutex_unlock(void *lock) "unlocked mutex %p"
+qemu_mutex_locked(void *lock) "locked mutex %p"
+qemu_mutex_unlocked(void *lock) "unlocked mutex %p"