[PATCH 6.6.y 1/2] srcu: Add NUM_ACTIVE_SRCU_POLL_OLDSTATE

[Sasha Levin <sashal@kernel.org>]

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit 395e73bd8d35fa9b8505e44f63a2a10abde09cce ]

This commit adds NUM_ACTIVE_SRCU_POLL_OLDSTATE, which gives the maximum
number of distinct return values from get_state_synchronize_rcu()
that can, at a given point in time, correspond to not-completed SRCU
grace periods.

Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Closes: https://lore.kernel.org/all/irycqy4sinjdgm2hkyix2bffunpcmuwgeufsx6nlljvqme3wiu@ify3zdnrmzph/
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Stable-dep-of: d06917d0ba22 ("srcu: Add list_splice_init_srcu()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/srcu.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 236610e4a8fa5..f664cba7a80c2 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -61,6 +61,10 @@ unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
 unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
 bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
 
+// Maximum number of unsigned long values corresponding to
+// not-yet-completed SRCU grace periods.
+#define NUM_ACTIVE_SRCU_POLL_OLDSTATE 2
+
 #ifdef CONFIG_NEED_SRCU_NMI_SAFE
 int __srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp);
 void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx) __releases(ssp);
-- 
2.53.0