Re: linux-next: cgroup_mount() falls asleep forever

From: Tejun Heo
Date: Wed Sep 24 2014 - 15:25:07 EST

Hey, Al.

On Wed, Sep 24, 2014 at 07:52:14PM +0100, Al Viro wrote:
> On Wed, Sep 24, 2014 at 06:29:27PM +0400, Andrey Wagin wrote:
> > 2014-09-24 14:31 GMT+04:00 Andrey Wagin <avagin@xxxxxxxxx>:
> > > Hi All,
> >
> > The problem is in a following commit:
> >
> > commit 0c7bf3e8cab7900e17ce7f97104c39927d835469
> > Author: Zefan Li <lizefan@xxxxxxxxxx>
> > 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@xxxxxxxxxx>
> > Signed-off-by: Zefan Li <lizefan@xxxxxxxxxx>
> > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

I'm gonna wait for Li's response for a couple days and then revert it
if it can't be fixed differently.

> 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.

Yeah, it's an ugly thing to work around vfs interface not very
conducive for filesystems which conditionally create or reuse
superblocks during mount. There was a thread explaining what's going
on. Looking up...


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