linux-kernel - Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation

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Date:   Wed, 17 Apr 2019 09:35:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to
 prevent lock starvation

On Tue, Apr 16, 2019 at 02:16:11PM -0400, Waiman Long wrote:

> >> @@ -324,6 +364,12 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
> >>  		adjustment -= RWSEM_FLAG_WAITERS;
> >>  	}
> >>  
> >> +	/*
> >> +	 * Clear the handoff flag
> >> +	 */
> > Right, but that is a trivial comment in the 'increment i' style, it
> > clearly states what the code does, but completely fails to elucidate the
> > code.
> >
> > Maybe:
> >
> > 	/*
> > 	 * When we've woken a reader, we no longer need to force writers
> > 	 * to give up the lock and we can clear HANDOFF.
> > 	 */
> >
> > And I suppose this is required if we were the pickup of the handoff set
> > above, but is there a guarantee that the HANDOFF was not set by a
> > writer?
> 
> I can change the comment. The handoff bit is always cleared in
> rwsem_try_write_lock() when the lock is successfully acquire. Will add a
> comment to document that.

That doesn't help much, because it drops ->wait_lock between setting it
and acquiring it. So the read-acquire can interleave.

I _think_ it works, but I'm having trouble explaining how exactly. I
think because readers don't spin yet and thus wakeups abide by queue
order.

And the other way around should have (write) spinners terminate the
moment they see HANDOFF set by a readers, but I'm not immediately seeing
that either.

I'll continue staring at that.