On Tue, Jan 27, 2015 at 10:09:18AM +0000, David Laight wrote:

But if err is non-zero you don't hold the mutex.
Presumably the calling code also errors out immediately,
so the RCU lock isn't needed at all.

No the calling code expects the RCU lock to be held because it
will always call stop.  Look at how seq_file works.

Cheers,
-- 
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