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Subject[tip: locking/core] futex: Add mutex around futex exit
The following commit has been merged into the locking/core branch of tip:

Commit-ID: 599ab3f76ebd32a027b95429f47dea739a7458fd
Gitweb: https://git.kernel.org/tip/599ab3f76ebd32a027b95429f47dea739a7458fd
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Wed, 06 Nov 2019 22:55:44 +01:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Fri, 15 Nov 2019 19:10:53 +01:00

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

---
include/linux/futex.h | 1 +
include/linux/sched.h | 1 +
kernel/futex.c | 16 ++++++++++++++++
3 files changed, 18 insertions(+)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 9daf8d9..214284c 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -63,6 +63,7 @@ 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
--- 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
--- 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)
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