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

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Date:   Mon, 10 Jan 2022 19:45:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH] f2fs: move f2fs to use reader-unfair rwsems

On Mon, Jan 10, 2022 at 11:18:27AM -0500, Waiman Long wrote:
> 
> On 1/10/22 03:05, 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.
> > > 
> > > Signed-off-by: Tim Murray <timmurray@...gle.com>
> > > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> 
> We could certainly implement a down_read() variant (e.g.
> down_read_lowprio()) with its own slowpath function to do this within the
> rwsem code as long as there is a good use-case for this kind of
> functionality.

I think _unfair() or something along those lines is a *much* better
naming that _lowprio(). Consider a RT task ending up calling _lowprio().
That just doesn't make conceptual sense.

And then there's the lockdep angle; the thing being unfair will lead to
scenarios where lockdep will give a false positive because it expects
the r-w-r order to block things, which won't happen. A position needs to
be taken a-prioriy.

But before all that, a sane problem description. Can't propose solutions
without having a problem.