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

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 9f27924..5cc3fed 100644 (file)
--- a/include/linux/futex.h
+++ b/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);

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 85dab2f..1ebe540 100644 (file)
--- 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 (file)
--- 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)