From 3f186d974826847a07bc7964d79ec4eded475ad9 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 6 Nov 2019 22:55:44 +0100
Subject: [PATCH 09/11] futex: Add mutex around futex exit
Git-commit: 3f186d974826847a07bc7964d79ec4eded475ad9
Patch-mainline: v5.5-rc1
References: bsc#1149032
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: Davidlohr Bueso <dbueso@suse.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 9f2792427d64..5cc3fed27d4c 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -65,6 +65,7 @@ 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 85dab2f721c9..1ebe540f8a08 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 0c9850af2724..46a81e611065 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3698,11 +3698,22 @@ 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.
*
@@ -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)
--
2.16.4