linux-kernel - Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

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Message-ID: <20151013204421.GN17308@twins.programming.kicks-ass.net>
Date:	Tue, 13 Oct 2015 22:44:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

On Tue, Oct 13, 2015 at 04:41:41PM -0400, Waiman Long wrote:
> On 10/13/2015 02:23 PM, Peter Zijlstra wrote:
> >On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote:
> >>  	for (;; waitcnt++) {
> >>+		loop = SPIN_THRESHOLD;
> >>+		while (loop) {
> >>+			/*
> >>+			 * Spin until the lock is free
> >>+			 */
> >>+			for (; loop&&  READ_ONCE(l->locked); loop--)
> >>+				cpu_relax();
> >>+			/*
> >>+			 * Seeing the lock is free, this queue head vCPU is
> >>+			 * the rightful next owner of the lock. However, the
> >>+			 * lock may have just been stolen by another task which
> >>+			 * has entered the slowpath. So we need to use atomic
> >>+			 * operation to make sure that we really get the lock.
> >>+			 * Otherwise, we have to wait again.
> >>+			 */
> >>+			if (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)
> >>+				goto gotlock;
> >>  		}
> >		for (loop = SPIN_THRESHOLD; loop; --loop) {
> >			if (!READ_ONCE(l->locked)&&
> >			cmpxchg(&l->locked, 0, _Q_LOCKED_VA) == 0)
> >				goto gotlock;
> >
> >			cpu_relax();
> >		}
> >
> 
> This was the code that I used in my original patch, but it seems to confuse
> you about doing too many lock stealing. So I separated it out to make my
> intention more explicit. I will change it back to the old code.

Code should be compact; its the purpose of Changelogs and comments to
explain it if its subtle.

Here you made weird code and the comments still don't explain how its
starvation proof and the Changelog is almost empty of useful.
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