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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Neeraj Upadhyay <neeraju@codeaurora.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.9 403/757] rcu/tree: Force quiescent state on callback overload
Date:   Tue, 27 Oct 2020 14:50:53 +0100
Message-Id: <20201027135509.484474750@linuxfoundation.org>
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From: Neeraj Upadhyay <neeraju@codeaurora.org>

[ Upstream commit 9c39245382de4d52a122641952900709d4a9950b ]

On callback overload, it is necessary to quickly detect idle CPUs,
and rcu_gp_fqs_check_wake() checks for this condition.  Unfortunately,
the code following the call to this function does not repeat this check,
which means that in reality no actual quiescent-state forcing, instead
only a couple of quick and pointless wakeups at the beginning of the
grace period.

This commit therefore adds a check for the RCU_GP_FLAG_OVLD flag in
the post-wakeup "if" statement in rcu_gp_fqs_loop().

Fixes: 1fca4d12f4637 ("rcu: Expedite first two FQS scans under callback-overload conditions")
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f78ee759af9cb..388a2ad292bf4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1898,7 +1898,7 @@ static void rcu_gp_fqs_loop(void)
 			break;
 		/* If time for quiescent-state forcing, do it. */
 		if (!time_after(rcu_state.jiffies_force_qs, jiffies) ||
-		    (gf & RCU_GP_FLAG_FQS)) {
+		    (gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) {
 			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("fqsstart"));
 			rcu_gp_fqs(first_gp_fqs);
-- 
2.25.1