shrinking rcu_read_lock() scope in d_alloc_parallel()

	The current use of rcu_read_lock() uses in d_alloc_parallel()
is fairly opaque - the single large scope serves two purposes.

	We start with lookup in normal hash, and there rcu_read_lock()
scope puts __d_lookup_rcu() and subsequent lockref_get_not_dead() into
the same RCU read-side critical area.

	If no match is found, we proceed to lock the hash chain of
in-lookup hash and scan that for a match.  If we find a match, we want
to grab it and wait for lookup in progress to finish.  Since the bitlock
we use for these hash chains has to nest inside ->d_lock, we need to
unlock the chain first and use lockref_get_not_dead() on the match.
That has to be done without breaking the RCU read-side critical area,
and we use the same rcu_read_lock() scope to bridge over.

	The thing is, after having grabbed the reference (and it is
very unlikely to fail) we proceed to grab ->d_lock - d_wait_lookup()
and __d_lookup_unhash()/__d_wake_in_lookup_waiters() are using that for
serialization.	That makes lockref_get_not_dead() pointless - trying
to avoid grabbing ->d_lock for refcount increment, only to grab it
anyway immediately after that.	If we grab ->d_lock first and replace
lockref_get_not_dead() with direct check for sign and increment if
non-negative we can move rcu_read_unlock() to immediately after grabbing
->d_lock.  Moreover, we don't need the RCU read-side critical area to
be contiguous since before earlier __d_lookup_rcu() - we can just as
well terminate the earlier one ASAP and call rcu_read_lock() again only
after having found a match (if any) in the in-lookup hash chain.

	That makes the entire thing easier to follow and the purpose
of those rcu_read_lock() calls easier to describe - the first scope is
for __d_lookup_rcu() + lockref_get_not_dead(), the second one bridges
over from the bitlock scope to the ->d_lock scope on the match found in
in-lookup hash.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

0 files changed, 0 insertions, 0 deletions