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Date: Thu, 15 Oct 2015 16:44:43 -0400 From: Waiman Long <waiman.long@....com> To: Peter Zijlstra <peterz@...radead.org> 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 10/13/2015 04:44 PM, Peter Zijlstra wrote: > 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. You are right. The current patch can't guarantee that there will be no lock starvation. I will make some code changes to make sure that lock starvation won't happen. I will also try to clean up the code and the comments at the same time. Cheers, Longman -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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