Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid starvation

[Peter Zijlstra]

On Mon, Oct 17, 2016 at 02:45:50PM -0400, Waiman Long wrote:
> On 10/07/2016 10:52 AM, Peter Zijlstra wrote:
> >  /*
> >   * Actual trylock that will work on any unlocked state.
> >+ *
> >+ * When setting the owner field, we must preserve the low flag bits.
> >+ *
> >+ * Be careful with @handoff, only set that in a wait-loop (where you set
> >+ * HANDOFF) to avoid recursive lock attempts.
> >   */
> >-static inline bool __mutex_trylock(struct mutex *lock)
> >+static inline bool __mutex_trylock(struct mutex *lock, const bool handoff)
> >  {
> >  	unsigned long owner, curr = (unsigned long)current;
> >
> >  	owner = atomic_long_read(&lock->owner);
> >  	for (;;) { /* must loop, can race against a flag */
> >-		unsigned long old;
> >+		unsigned long old, flags = __owner_flags(owner);
> >+
> >+		if (__owner_task(owner)) {
> >+			if (handoff&&  unlikely(__owner_task(owner) == current)) {
> >+				/*
> >+				 * Provide ACQUIRE semantics for the lock-handoff.
> >+				 *
> >+				 * We cannot easily use load-acquire here, since
> >+				 * the actual load is a failed cmpxchg, which
> >+				 * doesn't imply any barriers.
> >+				 *
> >+				 * Also, this is a fairly unlikely scenario, and
> >+				 * this contains the cost.
> >+				 */
> 
> I am not so sure about your comment here. I guess you are referring to the
> atomic_long_cmpxchg_acquire below for the failed cmpxchg. However, it is
> also possible that the path can be triggered on the first round without
> cmpxchg. Maybe we can do a load_acquire on the owner again to satisfy this
> requirement without a smp_mb().

Yes, I refer to the atomic_long_cmpxchg_acquire() below. If that cmpxchg
fails, no barriers are implied.

Yes we could fix that, but that would make all cmpxchg_acquire() loops
more expensive. And only fixing the initial load doesn't help, since we
still need to deal with the cmpxchg case failing.

We _could_ re-issue the load I suppose, because since if owner==current,
nobody else is going to change it, but I feel slightly uneasy with that.
Also, its a relative slow path, so for now, lets keep it simple and
explicit like so.

> 
> >+				smp_mb(); /* ACQUIRE */
> >+				return true;
> >+			}
> >
> >-		if (__owner_task(owner))
> >  			return false;
> >+		}
> >
> >-		old = atomic_long_cmpxchg_acquire(&lock->owner, owner,
> >-						  curr | __owner_flags(owner));
> >+		/*
> >+		 * We set the HANDOFF bit, we must make sure it doesn't live
> >+		 * past the point where we acquire it. This would be possible
> >+		 * if we (accidentally) set the bit on an unlocked mutex.
> >+		 */
> >+		if (handoff)
> >+			flags&= ~MUTEX_FLAG_HANDOFF;
> >+
> >+		old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);
> >  		if (old == owner)
> >  			return true;
> >
> >
> 
> Other than that, the code is fine.
> 
> Reviewed-by: Waiman Long <Waiman.Long@xxxxxxx>

Thanks!