linux-kernel - [PATCH v2 -rcu] srcu: Use rcu_seq_done

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Message-ID: <20250219124309.463702-1-joelagnelf@nvidia.com>
Date: Wed, 19 Feb 2025 07:43:08 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: linux-kernel@...r.kernel.org,
	Lai Jiangshan <jiangshanlai@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Joel Fernandes <joelagnelf@...dia.com>,
	Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
	rcu@...r.kernel.org
Subject: [PATCH v2 -rcu] srcu: Use rcu_seq_done_exact() for polling API

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@....com>
Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
---
v1->v2: Resend with Neeraj review tag. Rebased on rcu/dev.

 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 d2a694944553..591371d62e89 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.
-- 
2.34.1