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 <ava...@gmail.com>: > > > Hi All, > > > > The problem is in a following commit: > > > > commit 0c7bf3e8cab7900e17ce7f97104c39927d835469 > > Author: Zefan Li <lize...@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 <t...@kernel.org> > > Signed-off-by: Zefan Li <lize...@huawei.com> > > Signed-off-by: Tejun Heo <t...@kernel.org>
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... http://thread.gmane.org/gmane.linux.kernel.containers/27623/focus=10635 Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
