linux-kernel - Re: [PATCH v4] locking/rwsem: Make handoff bit handling more consistent

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Message-ID: <fd999d01-167b-3233-d527-def3a1c79913@redhat.com>
Date:   Mon, 15 Nov 2021 19:09:03 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        mazhenhua <mazhenhua@...omi.com>,
        Hillf Danton <hdanton@...a.com>,
        Maria Yu <quic_aiquny@...cinc.com>
Subject: Re: [PATCH v4] locking/rwsem: Make handoff bit handling more
 consistent


On 11/15/21 18:07, Peter Zijlstra wrote:
> On Mon, Nov 15, 2021 at 05:29:10PM -0500, Waiman Long wrote:
>> The handoff_set flag can only be true for a first waiter. A random waiter in
>> the middle of a wait queue will never has this flag set.
>>
>> This flag is set in two places in rwsem_try_write_lock():
> Bah, I thought it would unconditionally propagate the bit from @count. I
> missed the early exit :/
>
I am going to restructure the code there to make it easier to see that 
only the first waiter will have this bit set to avoid this confusion.

Cheers,
Longman