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

[Suren Baghdasaryan]

On Thu, Nov 21, 2024 at 7:36 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Thu, Nov 21, 2024 at 7:23 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxx/
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