[PATCH -v4 6/7] locking: Update spin_unlock_wait users

[Peter Zijlstra]

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

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 ipc/sem.c          |    1 -
 kernel/exit.c      |    8 ++++++--
 kernel/task_work.c |    1 -
 3 files changed, 6 insertions(+), 4 deletions(-)

--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -282,7 +282,6 @@ static void sem_wait_array(struct sem_ar
 		sem = sma->sem_base + i;
 		spin_unlock_wait(&sem->lock);
 	}
-	smp_acquire__after_ctrl_dep();
 }
 
 /*
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -776,10 +776,14 @@ 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())) {
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -108,7 +108,6 @@ void task_work_run(void)
 		 * fail, but it can play with *work and other entries.
 		 */
 		raw_spin_unlock_wait(&task->pi_lock);
-		smp_mb();
 
 		do {
 			next = work->next;