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Date: Fri, 26 Aug 2016 00:23:28 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Jason Low <jason.low2@hpe.com>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Chris Wilson <chris@chris-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@ffwll.ch>
Subject: Re: [RFC][PATCH -v2 4/4] locking/mutex: Add lock handoff to avoid
 starvation
Message-ID: <20160825222328.GN10138@twins.programming.kicks-ass.net>
References: <20160825183734.113736626@infradead.org>
 <20160825184324.934871397@infradead.org>
 <57BF6A73.1050505@hpe.com>
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On Thu, Aug 25, 2016 at 06:00:19PM -0400, Waiman Long wrote:
> On 08/25/2016 02:37 PM, Peter Zijlstra wrote:
> >@@ -468,9 +496,12 @@ void __sched mutex_unlock(struct mutex *
> >  	DEBUG_LOCKS_WARN_ON(__mutex_owner(lock) != current);
> >  #endif
> >
> >-	owner = atomic_long_fetch_and(MUTEX_FLAGS,&lock->owner);
> >+	owner = atomic_long_read(&lock->owner);
> >+	if (!(owner&  MUTEX_FLAG_HANDOFF))
> >+		owner = atomic_long_fetch_and(MUTEX_FLAGS,&lock->owner);
> >+
> >  	if (__owner_flags(owner))
> >-		__mutex_unlock_slowpath(lock);
> >+		__mutex_unlock_slowpath(lock, owner);
> >  }
> >  EXPORT_SYMBOL(mutex_unlock);
> 
> I don't think the race condition is fixed when we don't make sure that lock
> handoff only happens from current=>new. The problem is due to the fact that
> the MUTEX_FLAG_HANDOFF check in the unlock fastpath isn't serialized by the
> wait_lock. As a result, it is possible that the owner is NULL while the
> HANDOFF bit is set. Or an optimistic spinner may have stolen the lock in the
> interim.

You're right.. I'll try again tomorrow.