linux-kernel - Re: [PATCH] f2fs: move f2fs to use reader-unfair rwsems

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Message-ID: <YdxKiVHBUYvVP7W2@hirez.programming.kicks-ass.net>
Date:   Mon, 10 Jan 2022 16:02:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jaegeuk Kim <jaegeuk@...nel.org>, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        Tim Murray <timmurray@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH] f2fs: move f2fs to use reader-unfair rwsems

On Mon, Jan 10, 2022 at 12:05:26AM -0800, Christoph Hellwig wrote:
> Adding the locking primitive maintainers to this patch adding open coded
> locking primitives..
> 
> On Sat, Jan 08, 2022 at 08:46:17AM -0800, Jaegeuk Kim wrote:
> > From: Tim Murray <timmurray@...gle.com>
> > 
> > f2fs rw_semaphores work better if writers can starve readers,
> > especially for the checkpoint thread, because writers are strictly
> > more important than reader threads. This prevents significant priority
> > inversion between low-priority readers that blocked while trying to
> > acquire the read lock and a second acquisition of the write lock that
> > might be blocking high priority work.

*groan*... that's nowhere near enough Changelog to justify any of this.
Because next is a whole series of patches making things even worse
because lockdep no longers works as expected on this custom thing.

Can we start by describing the actual problem?