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

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Message-ID: <20230119175818.2rc4uvxkr3o6u37o@offworld>
Date:   Thu, 19 Jan 2023 09:58:18 -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, Davidlohr Bueso wrote:

>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

Nevermind, this was the rtmutex, not the wait_lock, sorry for the noise.