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>

3 files changed, 6 insertions, 4 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index 84dff3df11a4..ae72b3cddc8d 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -282,7 +282,6 @@ static void sem_wait_array(struct sem_array *sma)
                 sem = sma->sem_base + i;
                 spin_unlock_wait(&sem->lock);
         }
-        smp_acquire__after_ctrl_dep();
 }
 
 /*
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())) {
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 53fa971d000d..6ab4842b00e8 100644
--- 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;