Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v8 17/19] locking/rwsem: Merge owner into count on x86-64
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
References: <20190520205918.22251-1-longman@redhat.com>
 <20190520205918.22251-18-longman@redhat.com>
 <20190604094537.GK3402@hirez.programming.kicks-ass.net>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <d03f319a-790c-3084-2908-76f44d3f41f5@redhat.com>
Date:   Tue, 4 Jun 2019 11:47:21 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.6.1
MIME-Version: 1.0
In-Reply-To: <20190604094537.GK3402@hirez.programming.kicks-ass.net>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

Cheers,
Longman