[RFC 1/4] futex: add op wake for a single key

[Pavel Begunkov <asml.silence@gmail.com>]

Add a new futex wake function futex_wake_op_single(), which works
similar to futex_wake_op() but only for a single futex address as it
takes too many arguments. Also export it and other functions that will
be used by io_uring.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 include/linux/futex.h | 15 ++++++++++
 kernel/futex.c        | 64 +++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 77 insertions(+), 2 deletions(-)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index b70df27d7e85..04d500ae5983 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -77,6 +77,10 @@ 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_wake_op_single(u32 __user *uaddr, int nr_wake, unsigned int op,
+			 bool shared, bool try);
+int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
+	       ktime_t *abs_time, u32 bitset);
 #else
 static inline void futex_init_task(struct task_struct *tsk) { }
 static inline void futex_exit_recursive(struct task_struct *tsk) { }
@@ -88,6 +92,17 @@ static inline long do_futex(u32 __user *uaddr, int op, u32 val,
 {
 	return -EINVAL;
 }
+static inline int futex_wake_op_single(u32 __user *uaddr, int nr_wake,
+				       unsigned int op, bool shared, bool try)
+{
+	return -EINVAL;
+}
+static inline int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
+			     ktime_t *abs_time, u32 bitset)
+{
+	return -EINVAL;
+}
+
 #endif
 
 #endif
diff --git a/kernel/futex.c b/kernel/futex.c
index 4938a00bc785..75dc600062a4 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1681,6 +1681,66 @@ static int futex_atomic_op_inuser(unsigned int encoded_op, u32 __user *uaddr)
 	}
 }
 
+int futex_wake_op_single(u32 __user *uaddr, int nr_wake, unsigned int op,
+			 bool shared, bool try)
+{
+	union futex_key key;
+	struct futex_hash_bucket *hb;
+	struct futex_q *this, *next;
+	int ret, op_ret;
+	DEFINE_WAKE_Q(wake_q);
+
+retry:
+	ret = get_futex_key(uaddr, shared, &key, FUTEX_WRITE);
+	if (unlikely(ret != 0))
+		return ret;
+	hb = hash_futex(&key);
+retry_private:
+	spin_lock(&hb->lock);
+	op_ret = futex_atomic_op_inuser(op, uaddr);
+	if (unlikely(op_ret < 0)) {
+		spin_unlock(&hb->lock);
+
+		if (!IS_ENABLED(CONFIG_MMU) ||
+		    unlikely(op_ret != -EFAULT && op_ret != -EAGAIN)) {
+			/*
+			 * we don't get EFAULT from MMU faults if we don't have
+			 * an MMU, but we might get them from range checking
+			 */
+			ret = op_ret;
+			return ret;
+		}
+		if (try)
+			return -EAGAIN;
+
+		if (op_ret == -EFAULT) {
+			ret = fault_in_user_writeable(uaddr);
+			if (ret)
+				return ret;
+		}
+		cond_resched();
+		if (shared)
+			goto retry;
+		goto retry_private;
+	}
+	if (op_ret) {
+		plist_for_each_entry_safe(this, next, &hb->chain, list) {
+			if (match_futex(&this->key, &key)) {
+				if (this->pi_state || this->rt_waiter) {
+					ret = -EINVAL;
+					break;
+				}
+				mark_wake_futex(&wake_q, this);
+				if (++ret >= nr_wake)
+					break;
+			}
+		}
+	}
+	spin_unlock(&hb->lock);
+	wake_up_q(&wake_q);
+	return ret;
+}
+
 /*
  * Wake up all waiters hashed on the physical page that is mapped
  * to this virtual address:
@@ -2680,8 +2740,8 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
 	return ret;
 }
 
-static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
-		      ktime_t *abs_time, u32 bitset)
+int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
+	       ktime_t *abs_time, u32 bitset)
 {
 	struct hrtimer_sleeper timeout, *to;
 	struct restart_block *restart;
-- 
2.31.1