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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.9 013/293] rcu: Make synchronize_rcu_mult()
 check for duplicates
Thread-Topic: [PATCH AUTOSEL for 4.9 013/293] rcu: Make synchronize_rcu_mult()
 check for duplicates
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Date:   Mon, 9 Apr 2018 00:22:58 +0000
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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

[ Upstream commit 68ab0b4263224157f4d0c0e42854169a183d7534 ]

Currently, doing synchronize_rcu_mult(call_rcu, call_rcu) might
(or might not) wait for two RCU grace periods.  One approach is
of course "don't do that!", but in CONFIG_PREEMPT=3Dn kernels,
synchronize_rcu_mult(call_rcu, call_rcu_sched) does exactly that.
This results in an ugly #ifdef in sched_cpu_deactivate().

This commit therefore makes __wait_rcu_gp() check for duplicates,
which in turn allows duplicates to be passed to synchronize_rcu_mult()
without risk of waiting twice on the same type of grace period.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 kernel/rcu/update.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 4f6db7e6a117..2164c9cc6fd8 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -346,6 +346,7 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func=
_t *crcu_array,
 		   struct rcu_synchronize *rs_array)
 {
 	int i;
+	int j;
=20
 	/* Initialize and register callbacks for each flavor specified. */
 	for (i =3D 0; i < n; i++) {
@@ -357,7 +358,11 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_fun=
c_t *crcu_array,
 		}
 		init_rcu_head_on_stack(&rs_array[i].head);
 		init_completion(&rs_array[i].completion);
-		(crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
+		for (j =3D 0; j < i; j++)
+			if (crcu_array[j] =3D=3D crcu_array[i])
+				break;
+		if (j =3D=3D i)
+			(crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
 	}
=20
 	/* Wait for all callbacks to be invoked. */
@@ -366,7 +371,11 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_fun=
c_t *crcu_array,
 		    (crcu_array[i] =3D=3D call_rcu ||
 		     crcu_array[i] =3D=3D call_rcu_bh))
 			continue;
-		wait_for_completion(&rs_array[i].completion);
+		for (j =3D 0; j < i; j++)
+			if (crcu_array[j] =3D=3D crcu_array[i])
+				break;
+		if (j =3D=3D i)
+			wait_for_completion(&rs_array[i].completion);
 		destroy_rcu_head_on_stack(&rs_array[i].head);
 	}
 }
--=20
2.15.1