[PATCH 4.14 195/209] futex: Add mutex around futex exit

Greg Kroah-Hartman
2019-12-04T18:10:03Z

From: Thomas Gleixner <tglx@linutronix.de>

commit 3f186d974826847a07bc7964d79ec4eded475ad9 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -68,6 +68,7 @@ static inline void futex_init_task(struc
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);
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -959,6 +959,7 @@ 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
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3721,12 +3721,23 @@ static void futex_cleanup(struct task_st
*/
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
@@ -3749,6 +3760,11 @@ static void futex_cleanup_end(struct tas
* 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)