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Re: kern/28448 (stackable filesystems locking breakage when looking up DOTDOT)

> On Sat, Jan 03, 2009 at 01:11:59PM +0900, YAMAMOTO Takashi wrote:
>  > > 2) the lower layer does not export a lock pointer, in which case the
>  > >    relock dance in the lower layer will not use the upper layer's
>  > >    locks
>  > 
>  > this PR was about 2).
>  > it can deadlock between the parent and child directories
>  > in the upper layer due to the lack of dotdot relock dances in
>  > the upper layer.  the relock dance in the lower layer doesn't
>  > make sense because it's done keeping the upper layer vnode locked.
> So (to clarify as much as possible) the case is:
>    a. locks are not shared between the upper and lower layers
>    b. we try to lookup .. on an upper layer directory UD
>    c. vfs locks the directory, which locks both UD and the lower-layer
>       directory LD corresponding to UD
>    d. vfs calls VOP_LOOKUP, which goes to layer_lookup, which goes to
>       the lower fs
>    e. the lookup op on the lower fs unlocks LD and locks LD's parent,
>       then returns LD's parent
>    f. layer_lookup gets LD's parent back and digs out the
>       corresponding upper vnode with layer_node_create
>    g. layer_node_create then locks vnode without first unlocking UD
>    h. this violates the locking order, because UD's parent is locked
>       after UD.
> Do we have any layered fses where both c. and d. happen? It seems to
> me, based on my perhaps too vague understanding of how the layering
> system works, that if you don't share locks you can't expect to share
> the fs namespace, meaning you can't expect to use the default
> layer_lookup. I suspect this is not the only problem that'll show up
> if someone tries.
> If that's supposed to be legal, then it's a problem, yes. Nice catch :-)

locks can be shared only when the lower vnode exports them
for the upper layer.  ie. a layered filesystem can't assume that
it can share locks.

> (although note that I'm planning to abolish the dotdot-dance)

good to hear.  thanks for working on it.
how will you do so?


> -- 
> David A. Holland

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