linux-kernel - [PATCH rcu 3/4] srcu: Make Tiny SRCU poll_state_synchronize

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Date:   Wed, 31 Aug 2022 11:14:34 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 3/4] srcu: Make Tiny SRCU poll_state_synchronize_srcu() more precise

This commit applies the more-precise grace-period-state check used by
rcu_seq_done_exact() to poll_state_synchronize_srcu().  This is important
because Tiny SRCU uses a 16-bit counter, which can wrap quite quickly.
If counter wrap continues to be a problem, then expanding ->srcu_idx
and ->srcu_idx_max to 32 bits might be warranted.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/srcutiny.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index 92c002d654828..a2af24f214676 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -240,10 +240,10 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
  */
 bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
 {
-	bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
+	unsigned short cur_s = READ_ONCE(ssp->srcu_idx);
 
 	barrier();
-	return ret;
+	return USHORT_CMP_GE(cur_s, cookie) || USHORT_CMP_LT(cur_s, cookie - 3);
 }
 EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
 
-- 
2.31.1.189.g2e36527f23