[tip: locking/core] futex: Add mutex around futex exit

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     599ab3f76ebd32a027b95429f47dea739a7458fd
Gitweb:        https://git.kernel.org/tip/599ab3f76ebd32a027b95429f47dea739a7458fd
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Wed, 06 Nov 2019 22:55:44 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Fri, 15 Nov 2019 19:10:53 +01:00

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@xxxxxxxxxxxxx>
Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20191106224556.845798895@xxxxxxxxxxxxx

---
 include/linux/futex.h |  1 +
 include/linux/sched.h |  1 +
 kernel/futex.c        | 16 ++++++++++++++++
 3 files changed, 18 insertions(+)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 9daf8d9..214284c 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -63,6 +63,7 @@ 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);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 85dab2f..1ebe540 100644
--- a/include/linux/sched.h
+++ b/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
diff --git a/kernel/futex.c b/kernel/futex.c
index 0c9850a..46a81e6 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3698,12 +3698,23 @@ 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.
 	 *
 	 * This ensures that all subsequent checks of tsk->futex_state in
@@ -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)