Re: [PATCH 03/13] srcutree: Add reader-free fastpath to synchronize_srcu_atomic()

[David Woodhouse]

On Tue, 2026-09-08 at 13:29 -0700, Paul E. McKenney wrote:
> On Mon, Sep 07, 2026 at 03:58:19PM +0800, Kunwu Chan wrote:
> > From: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> > 
> > synchronize_srcu_atomic() is restricted to srcu_read_lock_atomic() and
> > srcu_read_unlock_atomic(), whose read-side critical sections disable
> > preemption.  In the common case where there are no readers at all, the
> > grace period therefore need not do the index flip.  Add a fastpath
> > that sums both ranks of the per-CPU ->srcu_ctrs[] counters and, if the
> > lock counts match the unlock counts on both ranks, ends the grace
> > period immediately, skipping the srcu_advance_state() scans, mirroring
> > the similar Tiny SRCU fastpath.
> > 
> > Correctness requires the counter-sum proof to follow the grace-period
> > anchor written by srcu_gp_start(); placing it before the anchor could
> > let this grace period miss a pre-existing reader and return without
> > waiting for it.  The smp_mb() between the unlock and lock sums pairs
> > with the smp_mb() in __srcu_read_lock().  The grace period is ended
> > manually under ->lock and ->srcu_atomic_gp_flag.
> >
> > Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> 
> Adding David Woodhouse on CC.
> 
> David, does this provide appropriate performance in your use case?

Looks like it should; I've thrown it into the test. Thanks.

My version¹ didn't drive the GP at all, and just took the fast path if
there were no readers. And because it didn't drive the GP, I *think*
the "Correctness requires…" part of the commit message cited above
didn't apply?

Driving the GP does mean a certain amount of serialization that my
earlier bailout didn't incur, but I think that's probably only going to
show up on a microbenchmark. Let's see.

I *do* want the fast path used from synchronize_srcu_expedited()
though, if possible. That's what solves Sean's *other* problem. Can we
have that?

¹ https://git.infradead.org/?p=users/dwmw2/linux.git;a=commitdiff;h=0f1456f508
  https://git.infradead.org/?p=users/dwmw2/linux.git;a=commitdiff;h=62ff5ac4ca

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