Re: [RFC][PATCH 3/3] locking/mutex: Add lock handoff to avoid starvation

[Waiman Long]

On 08/23/2016 04:32 PM, Peter Zijlstra wrote:
> On Tue, Aug 23, 2016 at 03:47:53PM -0400, Waiman Long wrote:
> > On 08/23/2016 08:46 AM, Peter Zijlstra wrote:
> > > N
> > > @@ -573,8 +600,14 @@ __mutex_lock_common(struct mutex *lock,
> > >   		schedule_preempt_disabled();
> > >   		spin_lock_mutex(&lock->wait_lock, flags);
> > >
> > > +		if (__mutex_owner(lock) == current)
> > > +			break;
> > > +
> > >   		if (__mutex_trylock(lock))
> > >   			break;
> > > +
> > > +		if (__mutex_waiter_is_first(lock,&waiter))
> > > +			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
> > >   	}
> > >   	__set_task_state(task, TASK_RUNNING);
> > You may want to think about doing some spinning while the owner is active
> > instead of going back to sleep again here.
> For sure; I just didn't bother pulling in your patches. I didn't want to
> sink in more time in case people really hated on 1/3 ;-)

I think there is race in how the handoff is being done.

CPU 0                   CPU 1                   CPU 2

-----                   -----                   -----

__mutex_lock_common:                            mutex_optimistic_spin:

  __mutex_trylock()

                        mutex_unlock:

                          if (owner&  

                             MUTEX_FLAG_HANDOFF)

                          owner&= 0x3;

                                                   __mutex_trylock();

                                                     owner = CPU2;

  __mutex_set_flag(lock,

    MUTEX_FLAG_HANDOFF)

                        __mutex_unlock_slowpath:

                        __mutex_handoff:

                          owner = CPU0;


Now both CPUs 1 and 2 think they have the lock. One way to fix that is
to check if the owner is still the original lock holder (CPU 0) before
doing the handoff, like:

--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -97,6 +97,8 @@ static void __mutex_handoff(struct mutex *lock, struct 
task_st
        for (;;) {
                unsigned long old, new;

+               if ((owner & ~MUTEX_FLAG_ALL) != current)
+                       break;
                new = (owner & MUTEX_FLAG_WAITERS);
                new |= (unsigned long)task;

I also think that the MUTEX_FLAG_HANDOFF bit needs to be cleared if the list
is empty.

@@ -614,7 +633,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned
        mutex_remove_waiter(lock, &waiter, task);
        /* set it to 0 if there are no waiters left: */
        if (likely(list_empty(&lock->wait_list)))
-               __mutex_clear_flag(lock, MUTEX_FLAG_WAITERS);
+               __mutex_clear_flag(lock, 
MUTEX_FLAG_WAITERS|MUTEX_FLAG_HANDOFF);

Or we should try to reset the handoff bit after the while loop exit if 
the bit is still set.

Cheers,
Longman