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

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Date:   Tue, 15 May 2018 21:05:34 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Oleg Nesterov <oleg@...hat.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 Tue, May 15, 2018 at 9:02 PM, Waiman Long <longman@...hat.com> 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.
>

BUILD_BUG_ON(!rwsem_has_anonymous_owner(RWSEM_OWNER_UNKNOWN))
inside rwsem.c and comment above to explain it.

Thanks,
Amir.