Messages in this thread | | | Date | Wed, 24 Sep 2014 19:52:14 +0100 | From | Al Viro <> | Subject | Re: linux-next: cgroup_mount() falls asleep forever |
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On Wed, Sep 24, 2014 at 06:29:27PM +0400, Andrey Wagin wrote: > 2014-09-24 14:31 GMT+04:00 Andrey Wagin <avagin@gmail.com>: > > Hi All, > > The problem is in a following commit: > > commit 0c7bf3e8cab7900e17ce7f97104c39927d835469 > Author: Zefan Li <lizefan@huawei.com> > Date: Sat Sep 20 14:49:10 2014 +0800 > > cgroup: remove redundant variable in cgroup_mount() > > Both pinned_sb and new_sb indicate if a new superblock is needed, > so we can just remove new_sb. > > Note now we must check if kernfs_tryget_sb() returns NULL, because > when it returns NULL, kernfs_mount() may still re-use an existing > superblock, which is just allocated by another concurent mount. > > Suggested-by: Tejun Heo <tj@kernel.org> > Signed-off-by: Zefan Li <lizefan@huawei.com> > Signed-off-by: Tejun Heo <tj@kernel.org>
Lovely... First of all, that thing is obviously racy - there's nothing to prevent another mount happening between these two places. Moreover, kernfs_mount() calling conventions are really atrocious - pointer to bool just to indicate that superblock is new?
Could somebody explain WTF is the whole construction trying to do? Not to mention anything else, what *does* this pinning a superblock protect from? Suppose we have a superblock for the same root with non-NULL ns and _that_ gets killed. We get hit by the same percpu_ref_kill(&root->cgrp.self.refcnt); so what's the point of pinned_sb? Might as well have just bumped the refcount, superblock or no superblock. And no, delaying that kernfs_kill_sb() does you no good whatsoever - again, pinned_sb might have nothing to do with the superblock we are after.
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