futex: Add mutex around futex exit

The mutex will be used in subsequent changes to replace the busy looping of
a waiter when the futex owner is currently executing the exit cleanup to
prevent a potential live lock.
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/20191106224556.845798895@linutronix.de

include/linux/futex.h

@@ -65,6 +65,7 @@ static inline void futex_init_task(struct task_struct *tsk)
 	INIT_LIST_HEAD(&tsk->pi_state_list);
 	tsk->pi_state_cache = NULL;
 	tsk->futex_state = FUTEX_STATE_OK;
+	mutex_init(&tsk->futex_exit_mutex);
 }
 
 void futex_exit_recursive(struct task_struct *tsk);

include/linux/sched.h

@@ -1053,6 +1053,7 @@ struct task_struct {
 #endif
 	struct list_head		pi_state_list;
 	struct futex_pi_state		*pi_state_cache;
+	struct mutex			futex_exit_mutex;
 	unsigned int			futex_state;
 #endif
 #ifdef CONFIG_PERF_EVENTS

kernel/futex.c

@@ -3698,11 +3698,22 @@ static void futex_cleanup(struct task_struct *tsk)
  */
 void futex_exit_recursive(struct task_struct *tsk)
 {
+	/* If the state is FUTEX_STATE_EXITING then futex_exit_mutex is held */
+	if (tsk->futex_state == FUTEX_STATE_EXITING)
+		mutex_unlock(&tsk->futex_exit_mutex);
 	tsk->futex_state = FUTEX_STATE_DEAD;
 }
 
 static void futex_cleanup_begin(struct task_struct *tsk)
 {
+	/*
+	 * Prevent various race issues against a concurrent incoming waiter
+	 * including live locks by forcing the waiter to block on
+	 * tsk->futex_exit_mutex when it observes FUTEX_STATE_EXITING in
+	 * attach_to_pi_owner().
+	 */
+	mutex_lock(&tsk->futex_exit_mutex);
+
 	/*
 	 * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
 	 *
@@ -3726,6 +3737,11 @@ static void futex_cleanup_end(struct task_struct *tsk, int state)
 	 * take another loop until it becomes visible.
 	 */
 	tsk->futex_state = state;
+	/*
+	 * Drop the exit protection. This unblocks waiters which observed
+	 * FUTEX_STATE_EXITING to reevaluate the state.
+	 */
+	mutex_unlock(&tsk->futex_exit_mutex);
 }
 
 void futex_exec_release(struct task_struct *tsk)