On Sat, Mar 19, 2016 at 6:55 PM, Al Viro [off-list ref] wrote:

What would make unlazy_walk() fail?  And if it succeeds, you are not
in RCU mode anymore *without* restarting from scratch...

I don't see your point.

You don't want to be in RCU mode any more. You want to either succeed
or fail with ECHILD/ESTALE. Then, in the failure case, you go to the
thread.

What I meant by restarting was the restart that do_filp_open() does,
and there it just restarts with "op->lookup_flags", which has
RCU_LOOKUP still set, so it would just try to do the RCU lookup again.

But I actually notice now that Ben actually disabled that restart if
LOOKUP_RCU was set, so that ends up not even happening.

Anyway, I'm not saying it's polished and pretty. I think the changes
to do_filp_open() are a bit silly, and the code should just use
path_openat() directly. Possibly using a new helper (ie perhaps just
introduce a "rcu_filp_openat()" thing). But from a design perspective,
I think this all looks fine.

              Linus

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