Re: [PATCH v4 tip/perf/core 2/4] mm: Introduce mmap_lock_speculation_{begin|end}

Suren Baghdasaryan
2024-11-21T16:32:28Z

On Thu, Nov 21, 2024 at 7:36 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Thu, Nov 21, 2024 at 7:23 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Thu, Nov 21, 2024 at 03:44:42PM +0100, Peter Zijlstra wrote:
> >
> > > But perhaps it makes even more sense to add this functionality to
> > > seqcount itself. The same argument can be made for seqcount_mutex and
> > > seqcount_rwlock users.
> >
> > Something like so I suppose.
>
> Ok, let me put this all together. Thanks!

I posted the new version at
https://lore.kernel.org/all/20241121162826.987947-1-surenb@google.com/
The changes are minimal, only those requested by Peter, so hopefully
they can be accepted quickly.

>
> >
> > ---
> > diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> > index 5298765d6ca4..102afdf8c7db 100644
> > --- a/include/linux/seqlock.h
> > +++ b/include/linux/seqlock.h
> > @@ -318,6 +318,28 @@ SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
> > __seq; \
> > })
> >
> > +/**
> > + * raw_seqcount_try_begin() - begin a seqcount_t read critical section
> > + * w/o lockdep and w/o counter stabilization
> > + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
> > + *
> > + * Very like raw_seqcount_begin(), except it enables eliding the critical
> > + * section entirely if odd, instead of doing the speculation knowing it will
> > + * fail.
> > + *
> > + * Useful when counter stabilization is more or less equivalent to taking
> > + * the lock and there is a slowpath that does that.
> > + *
> > + * If true, start will be set to the (even) sequence count read.
> > + *
> > + * Return: true when a read critical section is started.
> > + */
> > +#define raw_seqcount_try_begin(s, start) \
> > +({ \
> > + start = raw_read_seqcount(s); \
> > + !(start & 1); \
> > +})
> > +
> > /**
> > * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
> > * lockdep and w/o counter stabilization