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[10/12] futex: Add mutex around futex exit

Message ID 20210201151214.2193508-11-lee.jones@linaro.org
State New
Headers show
Series Futex back-port from v4.9 | expand

Commit Message

Lee Jones Feb. 1, 2021, 3:12 p.m. UTC 
From: Thomas Gleixner <tglx@linutronix.de>


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>

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

-- 
2.25.1
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Patch

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)