linux-kernel - Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma

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Message-ID: <CAJuCfpHwmfauZihbexnSq9mEe3Yz0DxT=KwgBX7ATCr2QJp_pA@mail.gmail.com>
Date:   Tue, 17 Jan 2023 13:00:19 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, akpm@...ux-foundation.org,
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Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock

On Tue, Jan 17, 2023 at 12:31 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Tue 17-01-23 10:28:40, Suren Baghdasaryan wrote:
> [...]
> > > Then yes, that's a starvable lock.  Preventing starvation on the mmap
> > > sem was the original motivation for making rwsems non-starvable, so
> > > changing that behaviour now seems like a bad idea.  For efficiency, I'd
> > > suggest that a waiting writer set the top bit of the counter.  That way,
> > > all new readers will back off without needing to check a second variable
> > > and old readers will know that they *may* need to do the wakeup when
> > > atomic_sub_return_release() is negative.
> > >
> > > (rwsem.c has a more complex bitfield, but I don't think we need to go
> > > that far; the important point is that the waiting writer indicates its
> > > presence in the count field so that readers can modify their behaviour)
> >
> > Got it. Ok, I think we can figure something out to check if there are
> > waiting write-lockers and prevent new readers from taking the lock.
>
> Reinventing locking primitives is a ticket to weird bugs. I would stick
> with the rwsem and deal with performance fallouts after it is clear that
> the core idea is generally acceptable and based on actual real life
> numbers. This whole thing is quite big enough that we do not have to go
> through "is this new synchronization primitive correct and behaving
> reasonably" exercise.

Point taken. That's one of the reasons I kept this patch separate.
I'll drop this last patch from the series for now. One correction
though, this will not be a performance fallout but memory consumption
fallout.

>
> --
> Michal Hocko
> SUSE Labs