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 <[email protected]> > > > > 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 <[email protected]> > > 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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