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

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Date:   Mon, 10 Jan 2022 13:55:35 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 1/10/22 13:45, Peter Zijlstra wrote:
> 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.
I am fine with the _unfair() name as I am not good at naming:-)
>
> 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.

Right, we may need to twist lockdep to match the new behavior if we are 
going to provide such a functionality.

Cheers,
Longman