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Subject: Re: [PATCH v3 2/2] locking/percpu-rwsem: Annotate rwsem ownership
 transfer by setting RWSEM_OWNER_UNKNOWN
To: Matthew Wilcox <willy@infradead.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
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From: Waiman Long <longman@redhat.com>
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Date: Tue, 15 May 2018 14:45:12 -0400
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On 05/15/2018 02:02 PM, Matthew Wilcox wrote:
> On Tue, May 15, 2018 at 07:58:05PM +0200, 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'd rather change the definition to be ((struct task_struct *)2)
> otherwise this is both reader-owned and anonymously-owned which doesn't
> make much sense.

Thinking about it a bit more. I can actually just use one special bit
(bit 0) to designate an unknown owner. So for a reader-owned lock, it is
just owner == 1 as the owners are unknown for a reader owned lock. For a
lock owned by an unknown writer, it is (owner & 1) && (owner != 1). That
will justify the use of -1L and save bit 1 for future extension.

Cheers,
Longman