Messages in this thread | | | Date | Mon, 10 Jan 2022 19:45:15 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] f2fs: move f2fs to use reader-unfair rwsems |
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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@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. > > > > > > Signed-off-by: Tim Murray <timmurray@google.com> > > > Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.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.
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