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

[joserz@linux.vnet.ibm.com]

On Thu, Apr 27, 2017 at 10:55:04AM +0200, Paolo Bonzini wrote:
> 
> 
> 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.

I'm working on it
Thanks for your review!

> 
> 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"
>