linux-kernel - Re: [PATCH v3 2/2] locking/percpu-rwsem: Annotate rwsem ownership transfer by setting RWSEM_OWNER

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Message-ID: <fcdd9ebd-05b9-30b1-877a-db84aaa96770@redhat.com>
Date:   Tue, 15 May 2018 14:35:33 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Oleg Nesterov <oleg@...hat.com>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 2/2] locking/percpu-rwsem: Annotate rwsem ownership
 transfer by setting RWSEM_OWNER_UNKNOWN

On 05/15/2018 02:05 PM, Peter Zijlstra wrote:
> On Tue, May 15, 2018 at 02:02:00PM -0400, Waiman Long wrote:
>> On 05/15/2018 01:58 PM, Peter Zijlstra wrote:
>>> On Tue, May 15, 2018 at 01:38:04PM -0400, Waiman Long wrote:
>>>> +/*
>>>> + * Owner value to indicate the rwsem's owner is not currently known.
>>>> + */
>>>> +#define RWSEM_OWNER_UNKNOWN	((struct task_struct *)-1)
>>> It might be nice to comment that this works and relies on having that
>>> ANON_OWNER bit set.
>> I am just trying not to expose internal working of rwsem, but I can
>> document that one of the bits is the real deal without specifying which one.
> Thing is, you don't want someone changing this without knowing about
> that one magic bit. It doesn't hurt to be explicit here.
>
> Also, do we want -1L instead of a -1 literal?
>
Yes, I should -1L instead.

Cheers,
Longman