[PATCH] fs: fix a posible leak of allocated superblock

[Seth Forshee]

On Tue, Mar 15, 2016 at 03:08:50PM +0300, Pavel Tikhomirov wrote:
> We probably need to fix superblock leak in patch (v4 "fs: Add user
> namesapace member to struct super_block"):
> 
> Imagine posible code path in sget_userns: we iterate through
> type->fs_supers and do not find suitable sb, we drop sb_lock to
> allocate s and go to retry. After we dropped sb_lock some other
> task from different userns takes sb_lock, it is already in retry
> stage and has s allocated, so it puts its s in type->fs_supers
> list. So in retry we will find these sb in list and check it has
> a different userns, and finally we will return without freeing s.
> 
> Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

Good catch.

Acked-by: Seth Forshee <seth.forshee at canonical.com>