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Date:   Fri, 16 Dec 2022 16:03:16 +0100
From:   Jiri Wiesner <jwiesner@suse.de>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        linux-kernel@vger.kernel.org, john.p.donnelly@oracle.com,
        Hillf Danton <hdanton@sina.com>,
        Mukesh Ojha <quic_mojha@quicinc.com>,
        Ting11 Wang =?utf-8?B?546L5am3?= <wangting11@xiaomi.com>
Subject: Re: [PATCH v6 2/6] locking/rwsem: Disable preemption at all
 down_read*() and up_read() code paths
Message-ID: <20221216150316.GA19097@incl>
References: <20221118022016.462070-1-longman@redhat.com>
 <20221118022016.462070-3-longman@redhat.com>
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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@quicinc.com>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---

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

-- 
Jiri Wiesner
SUSE Labs