A lot of ->destroy_inode() instances end with call_rcu() of a callback
that does RCU-delayed part of freeing. Introduce a new method for
doing just that, with saner signature.
Rules:
->destroy_inode ->free_inode
f g immediate call of f(),
RCU-delayed call of g()
f NULL immediate call of f(),
no RCU-delayed calls
NULL g RCU-delayed call of g()
NULL NULL RCU-delayed default freeing
IOW, NULL ->free_inode gives the same behaviour as now.
Note that NULL, NULL is equivalent to NULL, free_inode_nonrcu; we could
mandate the latter form, but that would have very little benefit beyond
making rules a bit more symmetric. It would break backwards compatibility,
require extra boilerplate and expected semantics for (NULL, NULL) pair
would have no use whatsoever...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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