linux-kernel - [PATCH rcu v2] 07/20] srcu: Force synchronization for srcu_get

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Message-Id: <20250130190317.1652481-7-paulmck@kernel.org>
Date: Thu, 30 Jan 2025 11:03:04 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	syzbot+16a19b06125a2963eaee@...kaller.appspotmail.com,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	bpf@...r.kernel.org
Subject: [PATCH rcu v2] 07/20] srcu: Force synchronization for srcu_get_delay()

Currently, srcu_get_delay() can be called concurrently, for example,
by a CPU that is the first to request a new grace period and the CPU
processing the current grace period.  Although concurrent access is
harmless, it unnecessarily expands the state space.  Additionally,
all calls to srcu_get_delay() are from slow paths.

This commit therefore protects all calls to srcu_get_delay() with
ssp->srcu_sup->lock, which is already held on the invocation from the
srcu_funnel_gp_start() function.  While in the area, this commit also
adds a lockdep_assert_held() to srcu_get_delay() itself.

Reported-by: syzbot+16a19b06125a2963eaee@...kaller.appspotmail.com
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Andrii Nakryiko <andrii@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: <bpf@...r.kernel.org>
---
 kernel/rcu/srcutree.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index dfc98e69acca..46e4cdaa1786 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -648,6 +648,7 @@ static unsigned long srcu_get_delay(struct srcu_struct *ssp)
 	unsigned long jbase = SRCU_INTERVAL;
 	struct srcu_usage *sup = ssp->srcu_sup;
 
+	lockdep_assert_held(&ACCESS_PRIVATE(ssp->srcu_sup, lock));
 	if (srcu_gp_is_expedited(ssp))
 		jbase = 0;
 	if (rcu_seq_state(READ_ONCE(sup->srcu_gp_seq))) {
@@ -675,9 +676,13 @@ static unsigned long srcu_get_delay(struct srcu_struct *ssp)
 void cleanup_srcu_struct(struct srcu_struct *ssp)
 {
 	int cpu;
+	unsigned long delay;
 	struct srcu_usage *sup = ssp->srcu_sup;
 
-	if (WARN_ON(!srcu_get_delay(ssp)))
+	spin_lock_irq_rcu_node(ssp->srcu_sup);
+	delay = srcu_get_delay(ssp);
+	spin_unlock_irq_rcu_node(ssp->srcu_sup);
+	if (WARN_ON(!delay))
 		return; /* Just leak it! */
 	if (WARN_ON(srcu_readers_active(ssp)))
 		return; /* Just leak it! */
@@ -1101,7 +1106,9 @@ static bool try_check_zero(struct srcu_struct *ssp, int idx, int trycount)
 {
 	unsigned long curdelay;
 
+	spin_lock_irq_rcu_node(ssp->srcu_sup);
 	curdelay = !srcu_get_delay(ssp);
+	spin_unlock_irq_rcu_node(ssp->srcu_sup);
 
 	for (;;) {
 		if (srcu_readers_active_idx_check(ssp, idx))
@@ -1854,7 +1861,9 @@ static void process_srcu(struct work_struct *work)
 	ssp = sup->srcu_ssp;
 
 	srcu_advance_state(ssp);
+	spin_lock_irq_rcu_node(ssp->srcu_sup);
 	curdelay = srcu_get_delay(ssp);
+	spin_unlock_irq_rcu_node(ssp->srcu_sup);
 	if (curdelay) {
 		WRITE_ONCE(sup->reschedule_count, 0);
 	} else {
-- 
2.40.1