[PATCH] srcu: Add try_synchronize_srcu() for callers which can prove readers absent

From: David Woodhouse <[email protected]>

For a dedicated srcu_struct whose read-side critical sections are
short, atomic, and usually absent — such as one converted from a
spinning lock — the common case at synchronize time is that there are
no readers at all. Even synchronize_srcu_expedited() still costs the
caller an unconditional sleep and two trips through the SRCU workqueue
to discover that: tens of microseconds of machinery to wait for
nothing. And synchronize_srcu()'s auto-expedite heuristic explicitly
declines to expedite within exp_holdoff (25µs) of the previous grace
period, which is exactly the regime a burst of back-to-back
invalidations puts such a domain in.

Provide try_synchronize_srcu(), which proves the no-readers case
inline and returns true without sleeping, without the workqueue, and
without advancing the grace-period sequence (so cookies from
get_state_synchronize_srcu() and queued callbacks are unaffected). If
the proof fails, it returns false and the caller falls back:

	if (!try_synchronize_srcu(ssp))
		synchronize_srcu_expedited(ssp);

For Tree SRCU the proof sums both epochs' unlock counters, executes a
full barrier, then sums both epochs' lock counters. Equality proves a
moment within this function at which no reader existed: a reader
entering between the sums inflates only the lock sum (spurious
fallback, safe), and a reader whose increment is unobserved has not
yet returned from srcu_read_lock() — the barrier pairing with
__srcu_read_lock() guarantees such a reader sees every store the
caller made beforehand, so it is not a reader the caller is obliged
to wait for. Summing both epochs means no index flip is required, and
without a flip the counter-wrap concerns of
srcu_readers_active_idx_check() do not arise. Readers of the _fast()
flavors elide the read-side barrier this depends on; any sign of them
(in the unlock-side rdm mask, which is gathered unconditionally)
disqualifies the fast path.

For Tiny SRCU (!SMP) both nesting counts being zero already proves no
reader exists — a mid-section reader could only be preempted or in an
interrupt, either of which leaves its count visibly elevated — and
program order on the sole CPU provides all the required ordering.

The immediate motivation is a proposed conversion of KVM's
gfn_to_pfn_cache to use SRCU¹, where the mmu_notifier invalidation
path must drain readers of a cache (in the manner of a TLB shootdown)
before the primary MMU zaps the backing page. Those readers are short
non-sleeping fast paths, some in contexts which cannot sleep (hardirq
event channel delivery, the scheduler's sched-out hook); measurement
under a worst-case invalidation flood shows 98.8% of drains complete inline
in 4-16µs where the expedited grace period took 32-128µs, with the
wait dominated by workqueue round-trip latency, not by readers.

¹ https://lore.kernel.org/all/[email protected]/

More potential use cases already exist in the tree with the same
no-readers-common-case profile: kvm->irq_srcu takes half a dozen
expedited grace periods in the irqfd and routing-update paths (bursts
of which, at VM boot, fall inside the exp_holdoff window), and mshv's
pt_irq_srcu is the same shape.

Signed-off-by: David Woodhouse <[email protected]>
Assisted-by: Claude:claude-mythos-5
---
Compile-tested for Tiny SRCU (tinyconfig); the Tree version has had
the KVM gfn_to_pfn_cache conversion soaking on it under an adversarial
invalidation flood.

 include/linux/srcu.h  |  1 +
 kernel/rcu/srcutiny.c | 23 ++++++++++++++
 kernel/rcu/srcutree.c | 70 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 94 insertions(+)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index a54ce9e808b9..0d7543c7becc 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -91,6 +91,7 @@ void call_srcu(struct srcu_struct *ssp, struct rcu_head *head,
 		void (*func)(struct rcu_head *head));
 void cleanup_srcu_struct(struct srcu_struct *ssp);
 void synchronize_srcu(struct srcu_struct *ssp);
+bool try_synchronize_srcu(struct srcu_struct *ssp);
 
 #define SRCU_GET_STATE_COMPLETED 0x1
 
diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index a2e2d516e51b..f07159cae241 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -260,6 +260,29 @@ void synchronize_srcu(struct srcu_struct *ssp)
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);
 
+/**
+ * try_synchronize_srcu - inline grace period for a reader-free srcu_struct
+ * @ssp: srcu_struct with which to synchronize.
+ *
+ * If @ssp provably has no readers in either epoch, provide the
+ * synchronize_srcu() guarantee to the caller immediately, without
+ * sleeping. Returns true on success; on failure the caller must fall
+ * back to synchronize_srcu().
+ *
+ * On !SMP a reader can only be mid-critical-section if it was
+ * preempted (or is running in an interrupt which preempted us), in
+ * which case its nesting count is visibly non-zero. Both counts being
+ * zero therefore proves that no reader exists, and any reader which
+ * begins after this function returns will, by program order on this
+ * sole CPU, observe every store the caller made before calling it.
+ */
+bool try_synchronize_srcu(struct srcu_struct *ssp)
+{
+	return !READ_ONCE(ssp->srcu_lock_nesting[0]) &&
+	       !READ_ONCE(ssp->srcu_lock_nesting[1]);
+}
+EXPORT_SYMBOL_GPL(try_synchronize_srcu);
+
 /*
  * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
  */
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 2601566c254a..76a78336feaa 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1645,6 +1645,76 @@ void synchronize_srcu(struct srcu_struct *ssp)
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);
 
+/**
+ * try_synchronize_srcu - inline grace period for a reader-free srcu_struct
+ * @ssp: srcu_struct with which to synchronize.
+ *
+ * If @ssp provably has no readers in either epoch, provide the
+ * synchronize_srcu() guarantee to the caller immediately: without
+ * sleeping, without a trip through the SRCU workqueue, and without
+ * advancing the grace-period sequence. Returns true on success; on
+ * failure the caller must fall back to synchronize_srcu() or
+ * synchronize_srcu_expedited().
+ *
+ * This serves dedicated srcu_struct structures whose read-side critical
+ * sections are short, atomic, and usually absent — where even an
+ * expedited grace period costs two trips through the workqueue and an
+ * unconditional sleep of the caller, three orders of magnitude more
+ * than the check below.
+ *
+ * Only readers of the srcu_read_lock() and srcu_read_lock_nmisafe()
+ * flavors are compatible with this proof; if the _fast() flavors have
+ * ever been used on @ssp, this function always returns false.
+ */
+bool try_synchronize_srcu(struct srcu_struct *ssp)
+{
+	unsigned long unlocks0, unlocks1;
+	unsigned long rdm0, rdm1;
+
+	check_init_srcu_struct(ssp);
+
+	/*
+	 * Order the caller's prior stores before the counter reads below.
+	 * Pairs (store-buffering pattern) with the smp_mb() in
+	 * __srcu_read_lock(): any reader whose lock increment is not
+	 * observed by the sums below is guaranteed to observe, within its
+	 * critical section, every store the caller made before calling
+	 * this function.
+	 */
+	smp_mb();
+
+	unlocks0 = srcu_readers_unlock_idx(ssp, 0, &rdm0);
+	unlocks1 = srcu_readers_unlock_idx(ssp, 1, &rdm1);
+
+	/*
+	 * Reader flavors which elide the read-side smp_mb() that the
+	 * pairing above depends on cannot be proven absent this way;
+	 * they need a real grace period.
+	 */
+	if ((rdm0 | rdm1) & SRCU_READ_FLAVOR_SLOWGP)
+		return false;
+
+	/*
+	 * As in srcu_readers_active_idx_check(): ensure that a lock is
+	 * always counted if the corresponding unlock is counted, so that
+	 * a reader racing with these sums can only inflate the lock sum
+	 * and force the (safe) fallback. Summing both epochs means no
+	 * index flip is needed: a stable equality proves there was a
+	 * moment in this function at which no readers existed at all.
+	 */
+	smp_mb();
+
+	if (!srcu_readers_lock_idx(ssp, 0, false, unlocks0))
+		return false;
+	if (!srcu_readers_lock_idx(ssp, 1, false, unlocks1))
+		return false;
+
+	/* Order the caller's subsequent accesses after the proof. */
+	smp_mb();
+	return true;
+}
+EXPORT_SYMBOL_GPL(try_synchronize_srcu);
+
 /**
  * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
  * @ssp: srcu_struct to provide cookie for.
-- 
2.43.0