On Tue, 07 Jun 2011 07:46:37 +0200
Miklos Szeredi [off-list ref] wrote:

quoted

Either way, I don't think that the uglypatch expresses a full
understanding of te bug ;)

I don't see a better way, how would we make nrpages update atomically
wrt the radix-tree while using only RCU?

The question is, does it matter that those two can get temporarily out
of sync?

In case of inode eviction it does, not only because of that BUG_ON, but
because page reclaim must be somehow synchronised with eviction.
Otherwise it may access tree_lock on the mapping of an already freed
inode.

In other cases?  AFAICS it doesn't matter.  Most ->nrpages accesses
weren't under tree_lock before Nick's RCUification, so their use were
just optimization.   

Gee, we've made a bit of a mess here.

Rather than bodging around particualr codesites where that mess exposes
itself, how about we step back and work out what our design is here,
then implement it and check that all sites comply with it?

What is the relationship between the radix-tree and nrpages?  What are
the locking rules?  Can anyone come up with a one-sentence proposal?

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