Date:   Mon, 10 Jan 2022 16:02:33 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Jaegeuk Kim <jaegeuk@kernel.org>, linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net,
        Tim Murray <timmurray@google.com>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH] f2fs: move f2fs to use reader-unfair rwsems
Message-ID: <YdxKiVHBUYvVP7W2@hirez.programming.kicks-ass.net>
References: <20220108164617.3130175-1-jaegeuk@kernel.org>
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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@google.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?