locking/rwsem, fs: Use killable down_read() in iterate_dir()

There was mutex_lock_interruptible() initially, and it was changed
to rwsem, but there were not killable rwsem primitives that time.
>From commit 9902af79c01a:

    "The main issue is the lack of down_write_killable(), so the places
     like readdir.c switched to plain inode_lock(); once killable
     variants of rwsem primitives appear, that'll be dealt with"

Use down_read_killable() same as down_write_killable() in !shared
case, as concurrent inode_lock() may take much time, that may be
wanted to be interrupted by user.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: arnd@arndb.de
Cc: avagin@virtuozzo.com
Cc: davem@davemloft.net
Cc: fenghua.yu@intel.com
Cc: gorcunov@virtuozzo.com
Cc: heiko.carstens@de.ibm.com
Cc: hpa@zytor.com
Cc: ink@jurassic.park.msu.ru
Cc: mattst88@gmail.com
Cc: rientjes@google.com
Cc: rth@twiddle.net
Cc: schwidefsky@de.ibm.com
Cc: tony.luck@intel.com
Cc: viro@zeniv.linux.org.uk
Link: http://lkml.kernel.org/r/150670120820.23930.5455667921545937220.stgit@localhost.localdomain
Signed-off-by: Ingo Molnar <mingo@kernel.org>

fs/readdir.c

@@ -36,13 +36,12 @@ int iterate_dir(struct file *file, struct dir_context *ctx)
 	if (res)
 		goto out;
 
-	if (shared) {
-		inode_lock_shared(inode);
-	} else {
+	if (shared)
+		res = down_read_killable(&inode->i_rwsem);
+	else
 		res = down_write_killable(&inode->i_rwsem);
-		if (res)
-			goto out;
-	}
+	if (res)
+		goto out;
 
 	res = -ENOENT;
 	if (!IS_DEADDIR(inode)) {