locks: reinstate locks_delete_block optimization

There is measurable performance impact in some synthetic tests due to
commit 6d390e4b5d48 (locks: fix a potential use-after-free problem when
wakeup a waiter). Fix the race condition instead by clearing the
fl_blocker pointer after the wake_up, using explicit acquire/release
semantics.

This does mean that we can no longer use the clearing of fl_blocker as
the wait condition, so switch the waiters over to checking whether the
fl_blocked_member list_head is empty.

Reviewed-by: yangerkun <[e-mail]>
Reviewed-by: NeilBrown <[e-mail]>
Fixes: 6d390e4b5d48 (locks: fix a potential use-after-free problem when wakeup a waiter)
Signed-off-by: Jeff Layton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/cifs/file.c

···
 	rc = posix_lock_file(file, flock, NULL);
 	up_write(&cinode->lock_sem);
 	if (rc == FILE_LOCK_DEFERRED) {
-		rc = wait_event_interruptible(flock->fl_wait, !flock->fl_blocker);
+		rc = wait_event_interruptible(flock->fl_wait,
+					list_empty(&flock->fl_blocked_member));
 		if (!rc)
 			goto try_again;
 		locks_delete_block(flock);

fs/locks.c

···
 {
 	locks_delete_global_blocked(waiter);
 	list_del_init(&waiter->fl_blocked_member);
-	waiter->fl_blocker = NULL;
 }
 
 static void __locks_wake_up_blocks(struct file_lock *blocker)
···
 			waiter->fl_lmops->lm_notify(waiter);
 		else
 			wake_up(&waiter->fl_wait);
+
+		/*
+		 * The setting of fl_blocker to NULL marks the "done"
+		 * point in deleting a block. Paired with acquire at the top
+		 * of locks_delete_block().
+		 */
+		smp_store_release(&waiter->fl_blocker, NULL);
 	}
 }
 
···
 {
 	int status = -ENOENT;
 
+	/*
+	 * If fl_blocker is NULL, it won't be set again as this thread "owns"
+	 * the lock and is the only one that might try to claim the lock.
+	 *
+	 * We use acquire/release to manage fl_blocker so that we can
+	 * optimize away taking the blocked_lock_lock in many cases.
+	 *
+	 * The smp_load_acquire guarantees two things:
+	 *
+	 * 1/ that fl_blocked_requests can be tested locklessly. If something
+	 * was recently added to that list it must have been in a locked region
+	 * *before* the locked region when fl_blocker was set to NULL.
+	 *
+	 * 2/ that no other thread is accessing 'waiter', so it is safe to free
+	 * it.  __locks_wake_up_blocks is careful not to touch waiter after
+	 * fl_blocker is released.
+	 *
+	 * If a lockless check of fl_blocker shows it to be NULL, we know that
+	 * no new locks can be inserted into its fl_blocked_requests list, and
+	 * can avoid doing anything further if the list is empty.
+	 */
+	if (!smp_load_acquire(&waiter->fl_blocker) &&
+	    list_empty(&waiter->fl_blocked_requests))
+		return status;
+
 	spin_lock(&blocked_lock_lock);
 	if (waiter->fl_blocker)
 		status = 0;
 	__locks_wake_up_blocks(waiter);
 	__locks_delete_block(waiter);
+
+	/*
+	 * The setting of fl_blocker to NULL marks the "done" point in deleting
+	 * a block. Paired with acquire at the top of this function.
+	 */
+	smp_store_release(&waiter->fl_blocker, NULL);
 	spin_unlock(&blocked_lock_lock);
 	return status;
 }
···
 		error = posix_lock_inode(inode, fl, NULL);
 		if (error != FILE_LOCK_DEFERRED)
 			break;
-		error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
+		error = wait_event_interruptible(fl->fl_wait,
+					list_empty(&fl->fl_blocked_member));
 		if (error)
 			break;
 	}
···
 		error = posix_lock_inode(inode, &fl, NULL);
 		if (error != FILE_LOCK_DEFERRED)
 			break;
-		error = wait_event_interruptible(fl.fl_wait, !fl.fl_blocker);
+		error = wait_event_interruptible(fl.fl_wait,
+					list_empty(&fl.fl_blocked_member));
 		if (!error) {
 			/*
 			 * If we've been sleeping someone might have
···
 
 	locks_dispose_list(&dispose);
 	error = wait_event_interruptible_timeout(new_fl->fl_wait,
-						!new_fl->fl_blocker, break_time);
+					list_empty(&new_fl->fl_blocked_member),
+					break_time);
 
 	percpu_down_read(&file_rwsem);
 	spin_lock(&ctx->flc_lock);
···
 		error = flock_lock_inode(inode, fl);
 		if (error != FILE_LOCK_DEFERRED)
 			break;
-		error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
+		error = wait_event_interruptible(fl->fl_wait,
+				list_empty(&fl->fl_blocked_member));
 		if (error)
 			break;
 	}
···
 		error = vfs_lock_file(filp, cmd, fl, NULL);
 		if (error != FILE_LOCK_DEFERRED)
 			break;
-		error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
+		error = wait_event_interruptible(fl->fl_wait,
+					list_empty(&fl->fl_blocked_member));
 		if (error)
 			break;
 	}