sys_swapon: fix inode locking

A conflict between 52c5056 ("mm: swap: unlock swapfile inode mutex
before closing file on bad swapfiles") and 83ef99b ("sys_swapon:
remove did_down variable") caused a double unlock of the inode mutex
(once in bad_swap: before the filp_close, once at the end just before
returning).

The patch which added the extra unlock cleared did_down to avoid
unlocking twice, but the other patch removed the did_down variable.

To fix, set inode to NULL after the first unlock, since it will be used
after that point only for the final unlock.

While checking this patch, I found a path which could unlock without
locking, in case the same inode was added as a swapfile twice. To fix,
move the setting of the inode variable further down, to just before
claim_swapfile, which will lock the inode before doing anything else.

Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Eric B Munson <emunson@mgebm.net>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -2088,7 +2088,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 
 	p->swap_file = swap_file;
 	mapping = swap_file->f_mapping;
-	inode = mapping->host;
 
 	for (i = 0; i < nr_swapfiles; i++) {
 		struct swap_info_struct *q = swap_info[i];
@@ -2101,6 +2100,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		}
 	}
 
+	inode = mapping->host;
+	/* If S_ISREG(inode->i_mode) will do mutex_lock(&inode->i_mutex); */
 	error = claim_swapfile(p, inode);
 	if (unlikely(error))
 		goto bad_swap;
@@ -2187,8 +2188,10 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	spin_unlock(&swap_lock);
 	vfree(swap_map);
 	if (swap_file) {
-		if (inode && S_ISREG(inode->i_mode))
+		if (inode && S_ISREG(inode->i_mode)) {
 			mutex_unlock(&inode->i_mutex);
+			inode = NULL;
+		}
 		filp_close(swap_file, NULL);
 	}
 out: