futex: Sanitize exit state handling

commit 4a8e991b91aca9e20705d434677ac013974e0e30 upstream

Instead of having a smp_mb() and an empty lock/unlock of task::pi_lock move
the state setting into to the lock section.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.645603214@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 10 insertions, 7 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index f995270a238a..6f4b9bd7ac15 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3740,16 +3740,19 @@ void futex_exit_recursive(struct task_struct *tsk)
 
 void futex_exit_release(struct task_struct *tsk)
 {
-	tsk->futex_state = FUTEX_STATE_EXITING;
-	/*
-	 * Ensure that all new tsk->pi_lock acquisitions must observe
-	 * FUTEX_STATE_EXITING. Serializes against attach_to_pi_owner().
-	 */
-	smp_mb();
 	/*
-	 * Ensure that we must observe the pi_state in exit_pi_state_list().
+	 * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
+	 *
+	 * This ensures that all subsequent checks of tsk->futex_state in
+	 * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
+	 * tsk->pi_lock held.
+	 *
+	 * It guarantees also that a pi_state which was queued right before
+	 * the state change under tsk->pi_lock by a concurrent waiter must
+	 * be observed in exit_pi_state_list().
 	 */
 	raw_spin_lock_irq(&tsk->pi_lock);
+	tsk->futex_state = FUTEX_STATE_EXITING;
 	raw_spin_unlock_irq(&tsk->pi_lock);
 
 	futex_exec_release(tsk);