linux-kernel - Re: [Question]: try to fix contention between expire_timers and try_to_del_timer

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Message-ID: <20170728092811.33bhkylg7kk6szxh@hirez.programming.kicks-ass.net>
Date:   Fri, 28 Jul 2017 11:28:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vikram Mulukutla <markivx@...eaurora.org>
Cc:     qiaozhou <qiaozhou@...micro.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>, sboyd@...eaurora.org,
        LKML <linux-kernel@...r.kernel.org>,
        Wang Wilbur <wilburwang@...micro.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel-owner@...r.kernel.org, sudeep.holla@....com
Subject: Re: [Question]: try to fix contention between expire_timers and
 try_to_del_timer_sync

On Thu, Jul 27, 2017 at 06:10:34PM -0700, Vikram Mulukutla wrote:

> I think we should have this discussion now - I brought this up earlier [1]
> and I promised a test case that I completely forgot about - but here it
> is (attached). Essentially a Big CPU in an acquire-check-release loop
> will have an unfair advantage over a little CPU concurrently attempting
> to acquire the same lock, in spite of the ticket implementation. If the Big
> CPU needs the little CPU to make forward progress : livelock.

This needs to be fixed in hardware. There really isn't anything the
software can sanely do about it.

It also doesn't have anything to do with the spinlock implementation.
Ticket or not, its a fundamental problem of LL/SC. Any situation where
we use atomics for fwd progress guarantees this can happen.

The little core (or really any core) should hold on to the locked
cacheline for a while and not insta relinquish it. Giving it a chance to
reach the SC.