locks: use cmpxchg to assign i_flctx pointer

During the v3.20/v4.0 cycle, I had originally had the code manage the
inode->i_flctx pointer using a compare-and-swap operation instead of the
i_lock.

Sasha Levin though hit a problem while testing with trinity that made me
believe that that wasn't safe. At the time, changing the code to protect
the i_flctx pointer seemed to fix the issue, but I now think that was
just coincidence.

The issue was likely the same race that Kirill Shutemov hit while
testing the pre-rc1 v4.0 kernel and that Linus spotted. Due to the way
that the spinlock was dropped in the middle of flock_lock_file, you
could end up with multiple flock locks for the same struct file on the
inode.

Reinstate the use of a CAS operation to assign this pointer since it's
likely to be more efficient and gets the i_lock completely out of the
file locking business.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>

fs/locks.c

@@ -223,14 +223,7 @@ locks_get_lock_context(struct inode *inode, int type)
 	 * Assign the pointer if it's not already assigned. If it is, then
 	 * free the context we just allocated.
 	 */
-	spin_lock(&inode->i_lock);
-	if (likely(!inode->i_flctx)) {
-		inode->i_flctx = new;
-		new = NULL;
-	}
-	spin_unlock(&inode->i_lock);
-
-	if (new)
+	if (cmpxchg(&inode->i_flctx, NULL, new))
 		kmem_cache_free(flctx_cache, new);
 out:
 	return inode->i_flctx;