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

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Message-ID: <20180515180219.GA6612@bombadil.infradead.org>
Date:   Tue, 15 May 2018 11:02:19 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.com>, 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>
Subject: Re: [PATCH v3 2/2] locking/percpu-rwsem: Annotate rwsem ownership
 transfer by setting RWSEM_OWNER_UNKNOWN

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.