linux-kernel - Re: [PATCH v6] locking/rwbase: Mitigate indefinite writer starvation.

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Message-ID: <20230322092848.hjlehutudsoz2hlz@techsingularity.net>
Date:   Wed, 22 Mar 2023 09:28:48 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linux-RT <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6] locking/rwbase: Mitigate indefinite writer starvation.

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@...hsingularity.net>
> Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

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

Thanks Sebastian and Thomas!

-- 
Mel Gorman
SUSE Labs