Messages in this thread | | | Date | Thu, 16 Jun 2022 10:02:22 -0700 | From | Jaegeuk Kim <> | Subject | Re: [GIT PULL] f2fs for 5.18 |
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On 06/15, Pavel Machek wrote: > Hi! > > > > > AFAICS, the read-unfair rwsem code is created to resolve a potential > > > > lock starvation problem that they found on linux-5.10.y stable tree. I > > > > believe I have fixed that in the v5.11 kernel, see commit 2f06f702925 > > > > ("locking/rwsem: Prevent potential lock starvation"). > > > > > > Ahh. > > > > > > Adding Tim Murray to the cc, since he was the source of that odd > > > reader-unfair thing. > > > > > > I really *really* dislike people thinking they can do locking > > > primitives, because history has taught us that they are wrong. > > > > > > Even when people get the semantics and memory ordering right (which is > > > not always the case, but at least the f2fs code uses real lock > > > primitives - just oddly - and should thus be ok), it invariably tends > > > to be a sign of something else being very wrong. > > > > > > And I can easily believe that in this case it's due to a rmsem issue > > > that was already fixed long long ago as per Waiman. > > > > > > Can people please test with the actual modern rwsem code and with the > > > odd reader-unfair locks disabled? > > > > The pain point is 1) we don't have a specific test to reproduce the issue, > > but got some foundings from field only, 2) in order to test the patches, we > > need to merge the patches into Android kernel [1] through LTS, 3) but, LTS > > wants to see any test results [2]. > > > > [1] https://android-review.googlesource.com/q/topic:rwsem_unfair > > [2] https://lore.kernel.org/stable/988fd9b5-8e89-03ae-3858-85320382792e@redhat.com/ > > Wait, what? Normally, patches are tested before going to mainline, and especially > before being backported to stable. > > If you can't reproduce issue on mainline kernel, there's something very wrong > with trying to fix it on mainline kernel. You should not be merging untested fixes > so that they can make it into mainline and then into stable and then into android kernel > to be tested.
What do you mean "untested fixes" here? As Tim mentioned [1], this F2FS patch resolved the issue in our Pixel devices.
[1] https://lore.kernel.org/lkml/CAEe=Sxmcn5+YUXBQhxDpzZVJu_T6S6+EURDqrP9uUS-PHGyuSg@mail.gmail.com/
> > If there's no other way, you should be able to backport those patches to android kernel and > test them _before_ merging them. Android phones are rather cheap. Some should even run mainline > kernels -- see for example Oneplus 4T -- if you don't need all the features.
IIUC, the point here was whether we need another generic rwsem API to address the issue or not. [1] said some rwsem fixes couldn't resolve our issue, and Waiman wanted to test another patch [2]. In order to avoid endless tests, I decided to get some results from Pixel using v5.15 (at least) by turning CONFIG_F2FS_UNFAIR_RWSEM off as of now. If we can see v5.15 works, I'm happy to revert the F2FS patch. Otherwise, we need it for our production.
[2] https://lore.kernel.org/lkml/5acaaf61-5419-178d-c805-62f979697653@redhat.com/#t
> > It looks hch was right NAKing the patches. > > Best regards, > > Pavel > -- > (english) http://www.livejournal.com/~pavelmachek > (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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