Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 22 Mar 2023 09:28:48 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linux-RT <linux-rt-users@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6] locking/rwbase: Mitigate indefinite writer starvation.
Message-ID: <20230322092848.hjlehutudsoz2hlz@techsingularity.net>
References: <20230321161140.HMcQEhHb@linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-15
Content-Disposition: inline
In-Reply-To: <20230321161140.HMcQEhHb@linutronix.de>
Precedence: bulk

On Tue, Mar 21, 2023 at 05:11:40PM +0100, Sebastian Andrzej Siewior wrote:
> The rw_semaphore and rwlock_t locks are unfair to writers. Readers can
> indefinitely acquire the lock unless the writer fully acquired the lock.
> This can never happen if there is always a reader in the critical
> section owning the lock.
> 
> Mel Gorman reported that since LTP-20220121 the dio_truncate test case
> went from having 1 reader to having 16 reader and the number of readers
> is sufficient to prevent the down_write ever succeeding while readers
> exist. Eventually the test is killed after 30 minutes as a failure.
> 
> Mel proposed a timeout to limit how long a writer can be blocked until
> the reader is forced into the slowpath.
> Thomas argued that there is no added value by providing this timeout.
> From PREEMPT_RT point of view, there are no critical rw_semaphore or
> rwlock_t locks left where the reader must be prefer.
> 

s/prefer/preferred/

> Mitigate indefinite writer starvation by forcing the READER into the
> slowpath once the WRITER attempts to acquire the lock.
> 
> Reported-by: Mel Gorman <mgorman@techsingularity.net>
> Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

Thanks Sebastian and Thomas!

-- 
Mel Gorman
SUSE Labs