linux-kernel - Re: [RFC PATCH-tip v4 07/10] locking/rwsem: Change RWSEM_WAITING

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Date:   Fri, 19 Aug 2016 12:21:57 -0400
From:   Waiman Long <waiman.long@....com>
To:     Wanpeng Li <kernellwp@...il.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        <linux-alpha@...r.kernel.org>, <linux-ia64@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Jason Low <jason.low2@...com>,
        Dave Chinner <david@...morbit.com>,
        Jonathan Corbet <corbet@....net>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v4 07/10] locking/rwsem: Change RWSEM_WAITING_BIAS
 for better disambiguation

On 08/19/2016 01:57 AM, Wanpeng Li wrote:
> 2016-08-19 5:11 GMT+08:00 Waiman Long<Waiman.Long@....com>:
>> When the count value is in between 0 and RWSEM_WAITING_BIAS, there
>> are 2 possibilities.
>> Either a writer is present and there is no waiter
> count = 0xffff0001
>
>> or there are waiters and readers. There is no easy way to
> count = 0xffff000X
>
> However, RWSEM_WAITING_BIAS is equal to 0xffff0000, so both these two
> cases are beyond RWSEM_WAITING_BIAS, right?
>
> Regards,
> Wanpeng Li

Perhaps I should make it clear that I am talking from a signed quantity 
point of view (it is an atomic_long_t). So

     RWSEM_WAITING_BIAS < RWSEM_ACTIVE_WRITE_BIAS < 0

Hope this clarify your question.

Cheers,
Longman