linux-kernel - Re: [RFC][PATCH -v2 4/4] locking/mutex: Add lock handoff to avoid starvation

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Date:   Thu, 25 Aug 2016 18:00:19 -0400
From:   Waiman Long <waiman.long@....com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Tim Chen" <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [RFC][PATCH -v2 4/4] locking/mutex: Add lock handoff to avoid
 starvation

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.

Cheers,
Longman