linux-kernel - Re: [PATCH v2] locking/rwbase: Prevent indefinite writer starvation

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Message-ID: <20230119174818.up7haooxje4nzusu@offworld>
Date:   Thu, 19 Jan 2023 09:48:18 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux-RT <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] locking/rwbase: Prevent indefinite writer starvation

On Thu, 19 Jan 2023, Mel Gorman wrote:

>The race could be closed by moving wait_lock acquisition before the
>atomic_sub in rwbase_write_lock() but it expands the scope of the wait_lock
>and I'm not sure that's necessary for either correctness or preventing
>writer starvation. It's a more straight-forward fix but expanding the
>scope of a lock unnecessarily has been unpopular in the past.

Curiously, this is the documented behavior:

  * down_write/write_lock()
  *  1) Lock rtmutex
  *  2) Remove the reader BIAS to force readers into the slow path