[PATCH 4/5] ipc/sem.c: Document and update memory barriers

[Manfred Spraul <manfred@colorfullife.com>]

The patch documents and updates the memory barriers in ipc/sem.c:
- Document that the WRITE_ONCE for q->status relies on a barrier
  inside wake_q_add().

- Read q->status using READ_ONCE+smp_acquire__after_ctrl_dep().
  as the pair for the barrier inside wake_q_add()

- Remove READ_ONCE & WRITE_ONCE for the situations where spinlocks
  provide exclusion.

- Add comments to all barriers, and mention the rules in the block
  regarding locking.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
---
 ipc/sem.c | 64 ++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 51 insertions(+), 13 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index ec97a7072413..53d970c4e60d 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -205,7 +205,9 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
  *
  * Memory ordering:
  * Most ordering is enforced by using spin_lock() and spin_unlock().
- * The special case is use_global_lock:
+ *
+ * Exceptions:
+ * 1) use_global_lock:
  * Setting it from non-zero to 0 is a RELEASE, this is ensured by
  * using smp_store_release().
  * Testing if it is non-zero is an ACQUIRE, this is ensured by using
@@ -214,6 +216,24 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
  * this smp_load_acquire(), this is guaranteed because the smp_load_acquire()
  * is inside a spin_lock() and after a write from 0 to non-zero a
  * spin_lock()+spin_unlock() is done.
+ *
+ * 2) queue.status:
+ * Initialization is done while holding sem_lock(), so no further barrier is
+ * required.
+ * Setting it to a result code is a RELEASE, this is ensured by both the
+ * barrier inside wake_q_add() (for case a) and while holding sem_lock()
+ * (for case b).
+ * The AQUIRE when reading the result code without holding sem_lock() is
+ * achieved by using READ_ONCE() + smp_acquire__after_ctrl_dep().
+ * (case a above).
+ * Reading the result code while holding sem_lock() needs no further barriers,
+ * the locks inside sem_lock() enforce ordering (case b above)
+ *
+ * 3) current->state:
+ * current->state is set to TASK_INTERRUPTIBLE while holding sem_lock().
+ * The wakeup is handled using the wake_q infrastructure. wake_q wakeups may
+ * happen immediately after calling wake_q_add. As wake_q_add() is called
+ * when holding sem_lock(), no further barriers are required.
  */
 
 #define sc_semmsl	sem_ctls[0]
@@ -766,13 +786,21 @@ static int perform_atomic_semop(struct sem_array *sma, struct sem_queue *q)
 static inline void wake_up_sem_queue_prepare(struct sem_queue *q, int error,
 					     struct wake_q_head *wake_q)
 {
+	/*
+	 * When the wakeup is performed, q->sleeper->state is read and later
+	 * set to TASK_RUNNING. This may happen at any time, even before
+	 * wake_q_add() returns. Memory ordering for q->sleeper->state is
+	 * enforced by sem_lock(): we own sem_lock now (that was the ACQUIRE),
+	 * and q->sleeper wrote q->sleeper->state before calling sem_unlock()
+	 * (->RELEASE).
+	 */
 	wake_q_add(wake_q, q->sleeper);
 	/*
-	 * Rely on the above implicit barrier, such that we can
-	 * ensure that we hold reference to the task before setting
-	 * q->status. Otherwise we could race with do_exit if the
-	 * task is awoken by an external event before calling
-	 * wake_up_process().
+	 * Memory barrier pairing:
+	 * case a: The barrier inside wake_q_add() pairs with
+	 *         READ_ONCE(q->status) + smp_acquire__after_ctrl_dep() in
+	 *         do_semtimedop().
+	 * case b: nothing, ordering is enforced by the locks in sem_lock().
 	 */
 	WRITE_ONCE(q->status, error);
 }
@@ -2148,9 +2176,11 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
 	}
 
 	do {
-		WRITE_ONCE(queue.status, -EINTR);
+		/* memory ordering ensured by the lock in sem_lock() */
+		queue.status = EINTR;
 		queue.sleeper = current;
 
+		/* memory ordering is ensured by the lock in sem_lock() */
 		__set_current_state(TASK_INTERRUPTIBLE);
 		sem_unlock(sma, locknum);
 		rcu_read_unlock();
@@ -2174,12 +2204,16 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
 		error = READ_ONCE(queue.status);
 		if (error != -EINTR) {
 			/*
-			 * User space could assume that semop() is a memory
-			 * barrier: Without the mb(), the cpu could
-			 * speculatively read in userspace stale data that was
-			 * overwritten by the previous owner of the semaphore.
+			 * Memory barrier for queue.status, case a):
+			 * The smp_acquire__after_ctrl_dep(), together with the
+			 * READ_ONCE() above pairs with the barrier inside
+			 * wake_q_add().
+			 * The barrier protects user space, too: User space may
+			 * assume that all data from the CPU that did the wakeup
+			 * semop() is visible on the wakee CPU when the sleeping
+			 * semop() returns.
 			 */
-			smp_mb();
+			smp_acquire__after_ctrl_dep();
 			goto out_free;
 		}
 
@@ -2189,7 +2223,11 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
 		if (!ipc_valid_object(&sma->sem_perm))
 			goto out_unlock_free;
 
-		error = READ_ONCE(queue.status);
+		/*
+		 * No necessity for any barrier:
+		 * We are protect by sem_lock() (case b)
+		 */
+		error = queue.status;
 
 		/*
 		 * If queue.status != -EINTR we are woken up by another process.
-- 
2.21.0