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Commit ad3466ae authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Greg Kroah-Hartman
Browse files

futex: Add mutex around futex exit 



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: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarIngo Molnar <mingo@kernel.org>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.845798895@linutronix.de


Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent ff3a33f3

include/linux/futex.h

+1 −0
Original line number Diff line number Diff line
@@ -70,6 +70,7 @@ 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);

include/linux/sched.h

+1 −0
Original line number Diff line number Diff line
@@ -1815,6 +1815,7 @@ 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

kernel/futex.c

+16 −0
Original line number Diff line number Diff line
@@ -3306,11 +3306,22 @@ 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.

	 *
@@ -3334,6 +3345,11 @@ 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)