[PATCH 4/4] srcu: Use rcu_seq_done_exact() for polling API

Joel Fernandes
2025-04-18T16:20:58Z

poll_state_synchronize_srcu() uses rcu_seq_done() unlike
poll_state_synchronize_rcu() which uses rcu_seq_done_exact().

The rcu_seq_done_exact() makes more sense for polling API, as with
this API, there is a higher chance that there is a significant delay
between the get_state..() and poll_state..() calls since a cookie
can be stored and reused at a later time. During such a delay, if
the gp_seq counter progresses more than ULONG_MAX/2 distance, then
poll_state..() may return false for a long time unwantedly.

Fix by using the more accurate rcu_seq_done_exact() API which is
exactly what straight RCU's polling does.

It may make sense, as future work, to add debug code here as well, where
we compare a physical timestamp between get_state..() and poll_state()
calls and yell if significant time has past but the grace period has
still not progressed.

Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
kernel/rcu/srcutree.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 9a59b071501b..48047260697e 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1589,7 +1589,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
{
if (cookie != SRCU_GET_STATE_COMPLETED &&
- !rcu_seq_done(&ssp->srcu_sup->srcu_gp_seq, cookie))
+ !rcu_seq_done_exact(&ssp->srcu_sup->srcu_gp_seq, cookie))
return false;
// Ensure that the end of the SRCU grace period happens before
// any subsequent code that the caller might execute.
--
2.43.0