linux-kernel - Re: [PATCH v5] locking/rwsem: Make handoff bit handling more consistent

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Date:   Wed, 17 Nov 2021 14:36:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
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 v5] locking/rwsem: Make handoff bit handling more
 consistent

On Mon, Nov 15, 2021 at 08:29:12PM -0500, Waiman Long wrote:
> There are some inconsistency in the way that the handoff bit is being
> handled in readers and writers.
> 
> Firstly, when a queue head writer set the handoff bit, it will clear it
> when the writer is being killed or interrupted on its way out without
> acquiring the lock. That is not the case for a queue head reader. The
> handoff bit will simply be inherited by the next waiter.
> 
> Secondly, in the out_nolock path of rwsem_down_read_slowpath(), both
> the waiter and handoff bits are cleared if the wait queue becomes empty.
> For rwsem_down_write_slowpath(), however, the handoff bit is not checked
> and cleared if the wait queue is empty. This can potentially make the
> handoff bit set with empty wait queue.
> 
> To make the handoff bit handling more consistent and robust, extract
> out handoff bit clearing code into the new rwsem_del_waiter() helper
> function.  The common function will only use atomic_long_andnot() to
> clear bits when the wait queue is empty to avoid possible race condition.
> If the first waiter with handoff bit set is killed or interrupted to
> exit the slowpath without acquiring the lock, the next waiter will
> inherit the handoff bit.
> 
> While at it, simplify the trylock for loop in rwsem_down_write_slowpath()
> to make it easier to read.
> 
> Fixes: 4f23dbc1e657 ("locking/rwsem: Implement lock handoff to prevent lock starvation")
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---

Zhenhua Ma, would you be able to confirm this works for you and provide
a Tested-by?