trace: add qemu mutex lock and unlock trace events

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>
Message-Id: <1493054398-26013-1-git-send-email-joserz@linux.vnet.ibm.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
[Also handle trylock, cond_wait and win32; trace "unlocked" while still
 in the critical section, so that "unlocked" always comes before the
 next "locked" tracepoint. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 17 insertions, 1 deletions

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 73e3a0edf5..eacd99e497 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -14,6 +14,7 @@
 #include "qemu/thread.h"
 #include "qemu/atomic.h"
 #include "qemu/notify.h"
+#include "trace.h"
 
 static bool name_threads;
 
@@ -60,17 +61,30 @@ void qemu_mutex_lock(QemuMutex *mutex)
     err = pthread_mutex_lock(&mutex->lock);
     if (err)
         error_exit(err, __func__);
+
+    trace_qemu_mutex_locked(mutex);
 }
 
 int qemu_mutex_trylock(QemuMutex *mutex)
 {
-    return pthread_mutex_trylock(&mutex->lock);
+    int err;
+
+    err = pthread_mutex_trylock(&mutex->lock);
+    if (err == 0) {
+        trace_qemu_mutex_locked(mutex);
+        return 0;
+    }
+    if (err != EBUSY) {
+        error_exit(err, __func__);
+    }
+    return -EBUSY;
 }
 
 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__);
@@ -130,7 +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__);
 }