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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
    http://lkml.org/lkml/2008/8/14/289
    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.

    Hugh


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