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authorThomas Gleixner2019-11-06 22:55:44 +0100
committerThomas Gleixner2019-11-20 09:40:10 +0100
commit3f186d974826847a07bc7964d79ec4eded475ad9 (patch)
tree99bdae5f1dedc0f361465fae87165eea0598eab4 /kernel/futex.c
parentaf8cbda2cfcaa5515d61ec500498d46e9a8247e2 (diff)
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
Diffstat (limited to 'kernel/futex.c')
-rw-r--r--kernel/futex.c16
1 files changed, 16 insertions, 0 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index 0c9850af2724..46a81e611065 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)