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

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Message-ID: <20161013151447.GA13138@arm.com>
Date:   Thu, 13 Oct 2016 16:14:47 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Waiman Long <waiman.long@....com>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid
 starvation

Hi Peter,

Just one comment below.

On Fri, Oct 07, 2016 at 04:52:48PM +0200, Peter Zijlstra wrote:
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -54,8 +54,10 @@ EXPORT_SYMBOL(__mutex_init);
>   * bits to store extra state.
>   *
>   * Bit0 indicates a non-empty waiter list; unlock must issue a wakeup.
> + * Bit1 indicates unlock needs to hand the lock to the top-waiter
>   */
>  #define MUTEX_FLAG_WAITERS	0x01
> +#define MUTEX_FLAG_HANDOFF	0x02
>  
>  #define MUTEX_FLAGS		0x03
>  
> @@ -71,20 +73,48 @@ static inline unsigned long __owner_flag
>  
>  /*
>   * 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.
> +				 */
> +				smp_mb(); /* ACQUIRE */

As we discussed on another thread recently, a failed cmpxchg_acquire
will always give you ACQUIRE semantics in practice. Maybe we should update
the documentation to allow this? The only special case is the full-barrier
version.

Will