linux-kernel - Re: [PATCH v6 2/6] locking/rwsem: Disable preemption at all down_read*() and up

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Message-ID: <20221216150316.GA19097@incl>
Date:   Fri, 16 Dec 2022 16:03:16 +0100
From:   Jiri Wiesner <jwiesner@...e.de>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, john.p.donnelly@...cle.com,
        Hillf Danton <hdanton@...a.com>,
        Mukesh Ojha <quic_mojha@...cinc.com>,
        Ting11 Wang 王婷 <wangting11@...omi.com>
Subject: Re: [PATCH v6 2/6] locking/rwsem: Disable preemption at all
 down_read*() and up_read() code paths

On Thu, Nov 17, 2022 at 09:20:12PM -0500, Waiman Long wrote:
> Commit 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically
> spin on owner") assumes that when the owner field is changed to NULL,
> the lock will become free soon. Commit 48dfb5d2560d ("locking/rwsem:
> Disable preemption while trying for rwsem lock") disables preemption
> when acquiring rwsem for write. However, preemption has not yet been
> disabled when acquiring a read lock on a rwsem. So a reader can add a
> RWSEM_READER_BIAS to count without setting owner to signal a reader,
> got preempted out by a RT task which then spins in the writer slowpath
> as owner remains NULL leading to live lock.
> 
> One easy way to fix this problem is to disable preemption at all the
> down_read*() and up_read() code paths as implemented in this patch.
> 
> Fixes: 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically spin on owner")
> Reported-by: Mukesh Ojha <quic_mojha@...cinc.com>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---

Tested-by: Jiri Wiesner <jwiesner@...e.de>

-- 
Jiri Wiesner
SUSE Labs