[PATCH AUTOSEL for 4.9 013/293] rcu: Make synchronize_rcu_mult() check for duplicates

[Sasha Levin]

From: "Paul E. McKenney" <paul...@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=n 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 <paul...@linux.vnet.ibm.com>
Signed-off-by: Sasha Levin <alexander.le...@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;
 
        /* Initialize and register callbacks for each flavor specified. */
        for (i = 0; i < n; i++) {
@@ -357,7 +358,11 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_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 = 0; j < i; j++)
+                       if (crcu_array[j] == crcu_array[i])
+                               break;
+               if (j == i)
+                       (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
        }
 
        /* Wait for all callbacks to be invoked. */
@@ -366,7 +371,11 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t 
*crcu_array,
                    (crcu_array[i] == call_rcu ||
                     crcu_array[i] == call_rcu_bh))
                        continue;
-               wait_for_completion(&rs_array[i].completion);
+               for (j = 0; j < i; j++)
+                       if (crcu_array[j] == crcu_array[i])
+                               break;
+               if (j == i)
+                       wait_for_completion(&rs_array[i].completion);
                destroy_rcu_head_on_stack(&rs_array[i].head);
        }
 }
-- 
2.15.1