locking/spinlock: Update spin_unlock_wait() users

With the modified semantics of spin_unlock_wait() a number of
explicit barriers can be removed. Also update the comment for the
do_exit() usecase, as that was somewhat stale/obscure.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 9e6e1356e6bb..0b40791b9e70 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -700,10 +700,14 @@ void do_exit(long code)
 
         exit_signals(tsk);  /* sets PF_EXITING */
         /*
-         * tsk->flags are checked in the futex code to protect against
-         * an exiting task cleaning up the robust pi futexes.
+         * Ensure that all new tsk->pi_lock acquisitions must observe
+         * PF_EXITING. Serializes against futex.c:attach_to_pi_owner().
          */
         smp_mb();
+        /*
+         * Ensure that we must observe the pi_state in exit_mm() ->
+         * mm_release() -> exit_pi_state_list().
+         */
         raw_spin_unlock_wait(&tsk->pi_lock);
 
         if (unlikely(in_atomic())) {