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

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Message-ID: <20190604094537.GK3402@hirez.programming.kicks-ass.net>
Date:   Tue, 4 Jun 2019 11:45:37 +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 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).