From 26f05b30e97d3bc30a20b52719d726f062539807 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Mon, 1 Feb 2021 15:12:12 +0000
Subject: [PATCH] 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: 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
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 805508373fcea..0f294ae63c78c 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -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);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index aba34bba5e9e3..8c10e97f94fea 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1704,6 +1704,7 @@ 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 feef5ce071aa5..d21b151216aa3 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3271,11 +3271,22 @@ 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.
*
@@ -3299,6 +3310,11 @@ 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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