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Subject: Re: [PATCH v3 2/2] locking/percpu-rwsem: Annotate rwsem ownership
 transfer by setting RWSEM_OWNER_UNKNOWN
To: Amir Goldstein <amir73il@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
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        Davidlohr Bueso <dave@stgolabs.net>,
        "Theodore Y. Ts'o" <tytso@mit.edu>, Oleg Nesterov <oleg@redhat.com>,
        Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>
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From: Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <e011289a-e13a-8839-7a48-eb6e58f5eaa7@redhat.com>
Date: Tue, 15 May 2018 14:45:57 -0400
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On 05/15/2018 02:05 PM, Amir Goldstein wrote:
> On Tue, May 15, 2018 at 9:02 PM, Waiman Long <longman@redhat.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.

Yes, it is a good idea. I will do that.

-Longman