Merge branch 'linus' into x86/asm, to pick up recent fixes

Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3d91b63dd2f6..0b6238f18da2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2016,6 +2016,28 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
         success = 1; /* we're going to change ->state */
         cpu = task_cpu(p);
 
+        /*
+         * Ensure we load p->on_rq _after_ p->state, otherwise it would
+         * be possible to, falsely, observe p->on_rq == 0 and get stuck
+         * in smp_cond_load_acquire() below.
+         *
+         * sched_ttwu_pending()                 try_to_wake_up()
+         *   [S] p->on_rq = 1;                  [L] P->state
+         *       UNLOCK rq->lock  -----.
+         *                              \
+         *                               +---   RMB
+         * schedule()                   /
+         *       LOCK rq->lock    -----'
+         *       UNLOCK rq->lock
+         *
+         * [task p]
+         *   [S] p->state = UNINTERRUPTIBLE     [L] p->on_rq
+         *
+         * Pairs with the UNLOCK+LOCK on rq->lock from the
+         * last wakeup of our task and the schedule that got our task
+         * current.
+         */
+        smp_rmb();
         if (p->on_rq && ttwu_remote(p, wake_flags))
                 goto stat;