From 3f186d974826847a07bc7964d79ec4eded475ad9 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Wed, 6 Nov 2019 22:55:44 +0100 Subject: 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 Reviewed-by: Ingo Molnar Acked-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20191106224556.845798895@linutronix.de --- kernel/futex.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) (limited to 'kernel/futex.c') diff --git a/kernel/futex.c b/kernel/futex.c index 0c9850af2724..46a81e611065 100644 --- a/kernel/futex.c +++ b/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) -- cgit v1.2.3