overlayfs, locking: Remove smp_mb__before_spinlock() usage

While we could replace the smp_mb__before_spinlock() with the new
smp_mb__after_spinlock(), the normal pattern is to use
smp_store_release() to publish an object that is used for
lockless_dereference() -- and mirrors the regular rcu_assign_pointer()
/ rcu_dereference() patterns.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

fs/overlayfs/readdir.c

@@ -446,14 +446,14 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end,
 
 			ovl_path_upper(dentry, &upperpath);
 			realfile = ovl_path_open(&upperpath, O_RDONLY);
-			smp_mb__before_spinlock();
+
 			inode_lock(inode);
 			if (!od->upperfile) {
 				if (IS_ERR(realfile)) {
 					inode_unlock(inode);
 					return PTR_ERR(realfile);
 				}
-				od->upperfile = realfile;
+				smp_store_release(&od->upperfile, realfile);
 			} else {
 				/* somebody has beaten us to it */
 				if (!IS_ERR(realfile))