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futex: Add mutex around futex exit

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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
This commit is contained in:
Thomas Gleixner 2019-11-06 22:55:44 +01:00
parent af8cbda2cf
commit 3f186d9748
3 changed files with 18 additions and 0 deletions
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1
include/linux/futex.h
View file

@ -65,6 +65,7 @@ static inline void futex_init_task(struct task_struct *tsk)
INIT_LIST_HEAD(&tsk->pi_state_list); INIT_LIST_HEAD(&tsk->pi_state_list);
tsk->pi_state_cache = NULL; tsk->pi_state_cache = NULL;
tsk->futex_state = FUTEX_STATE_OK; tsk->futex_state = FUTEX_STATE_OK;
mutex_init(&tsk->futex_exit_mutex);
} }
void futex_exit_recursive(struct task_struct *tsk); void futex_exit_recursive(struct task_struct *tsk);

1
include/linux/sched.h
View file

@ -1053,6 +1053,7 @@ struct task_struct {
#endif #endif
struct list_head pi_state_list; struct list_head pi_state_list;
struct futex_pi_state *pi_state_cache; struct futex_pi_state *pi_state_cache;
struct mutex futex_exit_mutex;
unsigned int futex_state; unsigned int futex_state;
#endif #endif
#ifdef CONFIG_PERF_EVENTS #ifdef CONFIG_PERF_EVENTS

16
kernel/futex.c
View file

@ -3698,11 +3698,22 @@ static void futex_cleanup(struct task_struct *tsk)
*/ */
void futex_exit_recursive(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; tsk->futex_state = FUTEX_STATE_DEAD;
} }
static void futex_cleanup_begin(struct task_struct *tsk) 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. * 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. * take another loop until it becomes visible.
*/ */
tsk->futex_state = state; 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) void futex_exec_release(struct task_struct *tsk)