[PATCH] dcache: avoid RCU for never-hashed dentries

[PATCH] dcache: avoid RCU for never-hashed dentries

Some dentries don't need to be globally visible in dentry hashtable.
(pipes & sockets)

Such dentries dont need to wait for a RCU grace period at delete time.
Being able to free them permits a better CPU cache use (hot cache)

This patch combined with (dont insert pipe dentries into dentry_hashtable)
reduced time of { pipe(p); close(p[0]); close(p[1]);} on my UP machine (1.6
GHz Pentium-M) from 3.23 us to 2.86 us (But this patch does not depend on
other patches, only bench results)

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Maneesh Soni <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Dipankar Sarma <[email protected]>
Acked-by: David Miller <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>