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SubjectRe: 2.6.27-rc7-sha1: EIP at proc_sys_compare+0x36/0x50
On Sun, 28 Sep 2008, Al Viro wrote:
> On Fri, Sep 26, 2008 at 08:47:51AM -0700, Linus Torvalds wrote:
> > and as far as I can tell, there is nothing to say that a /proc inode
> > cannot be a negative dentry. Sure, we try to get rid of them, but during a
> > parallel lookup, we will have added the dentry with a NULL inode in the
> > other lookup.
> >
> > So assuming that you have an inode at that point seems to be utter crap.
> >
> > Now, the whole _function_ is utter crap and should probably be dropped,
> > but whatever. That's just another sysctl insanity. In the meantime,
> > something like this does look appropriate, no?
> >
> > Al, did I miss something?
> The real underlying bug, whatever it is. If this sucker ever becomes
> negative, we have a big problem. Where _could_ that happen? Remember,
> we do not allow ->rmdir() and ->unlink() to succeed there. So d_delete()
> callers in namei.c are out of question. We also never do d_add() with
> NULL inode in there. We _might_ be doing a bogus d_rehash() on a negative
> /prooc/sys/<something> dentry that had never been hashed to start with
> somewhere in generic code, but... I don't see where that could happen.
> vfs_rename_dir() with negative new_dentry would have to get it from
> something and that would have to be ->lookup(). And that sucker returns
> ERR_PTR() or a positive dentry in all cases here. d_splice_alias() is not
> used there at all; d_move_locked() would scream bloody murder if dentry
> it's rehashing is negative. d_materialize_unique() and d_add_unique()
> are not used. So just WTF is creating this sucker?
> IOW, your patch will probably be enough to stop the visible problem, but
> I would dearly like to understand what's really causing it. It appears to
> be a refcounting breakage somewhere and we have *another* bug report that
> smells like that - it seems like we sometimes end up with negative dentry
> on alias list of an inode (outside of /proc/sys, AFAICT). Something really
> fishy is going on...

I got a couple of earlier instances of this on powerpc
but saw nothing more of it, so asked Al to forget about it.

But today I've got it again, this time on x86_64, with kdb in
(but not serial console), similar kernel builds with swapping
loads as before. Though with Andrew's latest mmotm, so some
details different from 2.6.27-rc, and could be an mmotm bug.

The dentry in question (it's for /proc/sys/kernel/ngroups_max)
looks as if the __d_drop and d_kill of prune_one_dentry() came
in on one cpu just after __d_lookup() had found the entry on
parent's hashlist, just before it acquired dentry->d_lock.

That's plausible, isn't it, and would account for the rarity,
and would say Linus's patch is good?

Do ask me for any details you'd like out of the dentry.


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