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

[Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>]

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 to see 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'd 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>
---
 util/qemu-thread-posix.c | 5 +++++
 util/trace-events        | 4 ++++
 2 files changed, 9 insertions(+)

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 73e3a0e..909c2ac 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;
 
@@ -60,6 +61,8 @@ void qemu_mutex_lock(QemuMutex *mutex)
     err = pthread_mutex_lock(&mutex->lock);
     if (err)
         error_exit(err, __func__);
+
+    trace_qemu_mutex_lock((void *)&mutex->lock);
 }
 
 int qemu_mutex_trylock(QemuMutex *mutex)
@@ -74,6 +77,8 @@ void qemu_mutex_unlock(QemuMutex *mutex)
     err = pthread_mutex_unlock(&mutex->lock);
     if (err)
         error_exit(err, __func__);
+
+    trace_qemu_mutex_unlock((void *)&mutex->lock);
 }
 
 void qemu_rec_mutex_init(QemuRecMutex *mutex)
diff --git a/util/trace-events b/util/trace-events
index b44ef4f..65c33fe 100644
--- a/util/trace-events
+++ b/util/trace-events
@@ -55,3 +55,7 @@ lockcnt_futex_wait_prepare(const void *lockcnt, int expected, int new) "lockcnt
 lockcnt_futex_wait(const void *lockcnt, int val) "lockcnt %p waiting on %d"
 lockcnt_futex_wait_resume(const void *lockcnt, int new) "lockcnt %p after wait: %d"
 lockcnt_futex_wake(const void *lockcnt) "lockcnt %p waking up one waiter"
+
+# util/qemu-thread-posix.c
+qemu_mutex_lock(void *qemu_global_mutex) "locked mutex %p"
+qemu_mutex_unlock(void *qemu_global_mutex) "unlocked mutex %p"
-- 
2.7.4