linux-kernel - Re: [PATCH v8 17/19] locking/rwsem: Merge owner into count on x86-64

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Date:   Tue, 4 Jun 2019 19:18:44 +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>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, 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 v8 17/19] locking/rwsem: Merge owner into count on x86-64

On Tue, Jun 04, 2019 at 01:06:11PM -0400, Waiman Long wrote:
> On 6/4/19 1:02 PM, Peter Zijlstra wrote:
> > On Tue, Jun 04, 2019 at 11:47:21AM -0400, Waiman Long wrote:
> >> On 6/4/19 5:45 AM, Peter Zijlstra wrote:
> >>> On Mon, May 20, 2019 at 04:59:16PM -0400, Waiman Long wrote:
> >>>> With separate count and owner, there are timing windows where the two
> >>>> values are inconsistent. That can cause problem when trying to figure
> >>>> out the exact state of the rwsem. For instance, a RT task will stop
> >>>> optimistic spinning if the lock is acquired by a writer but the owner
> >>>> field isn't set yet. That can be solved by combining the count and
> >>>> owner together in a single atomic value.
> >>> I just realized we can use cmpxchg_double() here (where available of
> >>> course).
> >> Does the 2 doubles need to be 128-bit aligned to use cmpxchg_double()? I
> >> don't think we can guarantee that unless we explicitly set this alignment.
> > It does :/ and yes, we'd need to play games with __align(2*sizeof(long))
> > and such.
> 
> So do you want this as an option now as it will be x86 specific? Or we
> can do that as a follow-up if we want to.

x86, s390 and arm64 have cmpxchg_double().

I was going to have a look (but like I wrote, I'm pretty useless today
so i didn't actually get anywhere) at the exact race that's a problem
here and see if there's not another solution too.