[PATCH v4 1/3] futex: Introduce function process_has_robust_futex()

[zhongjinji]

From: zhongjinji <zhongjinji@xxxxxxxxx>

When the holders of robust futexes are OOM killed but the waiters on robust
futexes are still alive, the robust futexes might be reaped before
futex_cleanup() runs. This can cause the waiters to block indefinitely [1].
To prevent this issue, the OOM reaper's work is delayed by 2 seconds [1].
However, the OOM reaper now rarely runs since many killed processes exit
within 2 seconds.

Because robust futex users are few, delay the reaper's execution only for
processes holding robust futexes to improve the performance of the OOM
reaper.

Introduce the function process_has_robust_futex() to detect whether a process
uses robust futexes. If each thread's robust_list in a process is NULL, it
means the process holds no robust futexes. Conversely, it means the process
holds robust futexes.

Link: https://lore.kernel.org/all/20220414144042.677008-1-npache@xxxxxxxxxx/T/#u [1]

Signed-off-by: zhongjinji <zhongjinji@xxxxxxxxx>
---
 include/linux/futex.h |  5 +++++
 kernel/futex/core.c   | 30 ++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 9e9750f04980..39540b7ae2a1 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -81,6 +81,7 @@ void futex_exec_release(struct task_struct *tsk);
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 	      u32 __user *uaddr2, u32 val2, u32 val3);
 int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4);
+bool process_has_robust_futex(struct task_struct *tsk);
 
 #ifdef CONFIG_FUTEX_PRIVATE_HASH
 int futex_hash_allocate_default(void);
@@ -108,6 +109,10 @@ static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsig
 {
 	return -EINVAL;
 }
+static inline bool process_has_robust_futex(struct task_struct *tsk)
+{
+	return false;
+}
 static inline int futex_hash_allocate_default(void)
 {
 	return 0;
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index d9bb5567af0c..01b6561ab4f6 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1961,6 +1961,36 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
 	return ret;
 }
 
+/*
+ * process_has_robust_futex() - check whether the given task hold robust futexes.
+ * @p: task struct of which task to consider
+ *
+ * If any thread in the task has a non-NULL robust_list or compat_robust_list,
+ * it indicates that the task holds robust futexes.
+ */
+bool process_has_robust_futex(struct task_struct *tsk)
+{
+	struct task_struct *t;
+	bool ret = false;
+
+	rcu_read_lock();
+	for_each_thread(tsk, t) {
+		if (unlikely(t->robust_list)) {
+			ret = true;
+			break;
+		}
+#ifdef CONFIG_COMPAT
+		if (unlikely(t->compat_robust_list)) {
+			ret = true;
+			break;
+		}
+#endif
+	}
+	rcu_read_unlock();
+
+	return ret;
+}
+
 static int __init futex_init(void)
 {
 	unsigned long hashsize, i;
-- 
2.17.1