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Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list is not NULL.

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On Fri, 2016-01-22 at 09:54 +0100, Peter Zijlstra wrote:
> On Thu, Jan 21, 2016 at 06:02:34PM -0500, Waiman Long wrote:
> > This patch attempts to fix this live-lock condition by enabling the
> > a woken task in the wait list to enter optimistic spinning loop itself
> > with precedence over the ones in the OSQ. This should prevent the
> > live-lock
> > condition from happening.
> 
> 
> So I think having the top waiter going back in to contend on the OSQ is
> an excellent idea, but I'm not sure the wlh_spinning thing is important.
> 
> The OSQ itself is FIFO fair, and the waiters retain the wait_list
> position. So having the top wait_list entry contending on the OSQ
> ensures we cannot starve (I think).

Right, and we can also avoid needing to add that extra field to the
mutex structure. Before calling optimistic spinning, we do want to check
if the lock is available to avoid unnecessary OSQ overhead though.

So maybe the following would be sufficient:

---
 kernel/locking/mutex.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 0551c21..ead0bd1 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -543,6 +543,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 	lock_contended(&lock->dep_map, ip);
 
 	for (;;) {
+		bool acquired = false;
+
 		/*
 		 * Lets try to take the lock again - this is needed even if
 		 * we get here for the first time (shortly after failing to
@@ -577,7 +579,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		/* didn't get the lock, go to sleep: */
 		spin_unlock_mutex(&lock->wait_lock, flags);
 		schedule_preempt_disabled();
+
+		if (mutex_is_locked(lock))
+			acquired = mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx);
 		spin_lock_mutex(&lock->wait_lock, flags);
+		if (acquired)
+			break;
 	}
 	__set_task_state(task, TASK_RUNNING);
 
-- 
1.7.2.5

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[PATCH RFC] locking/mutexes: don't spin on owner when wait list is  not NULL. Ding Tianhong <dingtianhong@huawei.com> - 2016-01-21 10:40 +0100
  Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Tim Chen <tim.c.chen@linux.intel.com> - 2016-01-21 22:30 +0100
    Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Davidlohr Bueso <dave@stgolabs.net> - 2016-01-22 03:50 +0100
      Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-22 04:20 +0100
    Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-22 03:50 +0100
  Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Waiman Long <waiman.long@hpe.com> - 2016-01-22 00:10 +0100
    Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Davidlohr Bueso <dave@stgolabs.net> - 2016-01-22 07:20 +0100
      Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Waiman Long <waiman.long@hpe.com> - 2016-01-22 14:40 +0100
        Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Davidlohr Bueso <dave@stgolabs.net> - 2016-01-22 17:50 +0100
          [PATCH] locking/mutex: Allow next waiter lockless wakeup Davidlohr Bueso <dave@stgolabs.net> - 2016-01-25 03:30 +0100
            Re: [PATCH] locking/mutex: Allow next waiter lockless wakeup Waiman Long <waiman.long@hpe.com> - 2016-01-26 00:10 +0100
    Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Peter Zijlstra <peterz@infradead.org> - 2016-01-22 10:00 +0100
      Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Jason Low <jason.low2@hp.com> - 2016-01-22 11:30 +0100
        Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Peter Zijlstra <peterz@infradead.org> - 2016-01-22 12:00 +0100
          Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Peter Zijlstra <peterz@infradead.org> - 2016-01-22 12:00 +0100
            Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Peter Zijlstra <peterz@infradead.org> - 2016-01-22 12:10 +0100
              Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Waiman Long <waiman.long@hpe.com> - 2016-01-22 15:10 +0100
                Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Ding Tianhong <dingtianhong@huawei.com> - 2016-01-24 09:10 +0100
      Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Waiman Long <waiman.long@hpe.com> - 2016-01-22 14:50 +0100

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