[tip:locking/core] sched/wait: Fix rcuwait_wake_up() ordering

[tip-bot for Prateek Sood]

Commit-ID:  6dc080eeb2ba01973bfff0d79844d7a59e12542e
Gitweb:     https://git.kernel.org/tip/6dc080eeb2ba01973bfff0d79844d7a59e12542e
Author:     Prateek Sood <prsood@xxxxxxxxxxxxxx>
AuthorDate: Fri, 30 Nov 2018 20:40:56 +0530
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 21 Jan 2019 11:15:36 +0100

sched/wait: Fix rcuwait_wake_up() ordering

For some peculiar reason rcuwait_wake_up() has the right barrier in
the comment, but not in the code.

This mistake has been observed to cause a deadlock in the following
situation:

    P1					P2

    percpu_up_read()			percpu_down_write()
      rcu_sync_is_idle() // false
					  rcu_sync_enter()
					  ...
      __percpu_up_read()

[S] ,-  __this_cpu_dec(*sem->read_count)
    |   smp_rmb();
[L] |   task = rcu_dereference(w->task) // NULL
    |
    |				    [S]	    w->task = current
    |					    smp_mb();
    |				    [L]	    readers_active_check() // fail
    `-> <store happens here>

Where the smp_rmb() (obviously) fails to constrain the store.

[ peterz: Added changelog. ]

Signed-off-by: Prateek Sood <prsood@xxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Davidlohr Bueso <dbueso@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Fixes: 8f95c90ceb54 ("sched/wait, RCU: Introduce rcuwait machinery")
Link: https://lkml.kernel.org/r/1543590656-7157-1-git-send-email-prsood@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/exit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 284f2fe9a293..3fb7be001964 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -307,7 +307,7 @@ void rcuwait_wake_up(struct rcuwait *w)
 	 *        MB (A)	      MB (B)
 	 *    [L] cond		  [L] tsk
 	 */
-	smp_rmb(); /* (B) */
+	smp_mb(); /* (B) */
 
 	/*
 	 * Avoid using task_rcu_dereference() magic as long as we are careful,