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 @@ -65,6 +65,7 @@ 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 @@ -1054,6 +1054,7 @@ 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 @@ -3698,12 +3698,23 @@ 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 @@ -3726,6 +3737,11 @@ 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)