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McKenney" , Akira Yokosawa , LKML Subject: Re: [PATCH V2] MIPS: implement smp_cond_load_acquire() for Loongson-3 Message-ID: <20180711110952.GC2476@hirez.programming.kicks-ass.net> References: <1531103198-16764-1-git-send-email-chenhc@lemote.com> <20180710121727.GK2476@hirez.programming.kicks-ass.net> <5471216.FKXZRxKFUI@flygoat-ry> <20180711102106.GG13963@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180711102106.GG13963@arm.com> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 11, 2018 at 11:21:06AM +0100, Will Deacon wrote: > On Wed, Jul 11, 2018 at 06:05:51PM +0800, Jiaxun Yang wrote: > > On 2018-7-10 Tue at 20:17:27，Peter Zijlstra Wrote： > > > Still, even with the rules above, the best work-around is still the very > > > same cpu_relax() hack. > > > > As you say, SFB makes Loongson not fully SMP-coherent. > > However, modify cpu_relax can solve the current problem, > > but not so straight forward. On the other hand, providing a Loongson-specific > > WRITE_ONCE looks more reasonable, because it the eliminate the "non-cohrency". > > So we can solve the bug from the root. > > Curious, but why is it not straight-forward to hack cpu_relax()? If you try > to hack WRITE_ONCE, you also need to hack atomic_set, atomic64_set and all > the places that should be using WRITE_ONCE but aren't ;~) Right. The problem isn't stores pre-se, normal progress should contain enough stores to flush out 'old' bits in the natural order of things. But the problem is spin-wait loops that inhibit normal progress (and thereby store-buffer flushing). And all spin-wait loops should be having cpu_relax() in them. So cpu_relax() is the natural place to fix this. Adding SYNC to WRITE_ONCE()/atomic* will hurt performance lots and will ultimately not guarantee anything more; and as Will said, keep you chasing dragons where people forgot to use WRITE_ONCE() where they maybe should've.