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/[email protected]/
>
> 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-phgy...@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/[email protected]/#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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