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During tbench/oprofile sessions, I found that dst_release() was in third position.

CPU: Core 2, speed 2999.68 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit mask of 0x00 (Unhalted core cycles) count 100000
samples  %        symbol name
483726    9.0185  __copy_user_zeroing_intel
191466    3.5697  __copy_user_intel
185475    3.4580  dst_release
175114    3.2648  ip_queue_xmit
153447    2.8608  tcp_sendmsg
108775    2.0280  tcp_recvmsg
102659    1.9140  sysenter_past_esp
101450    1.8914  tcp_current_mss
95067     1.7724  __copy_from_user_ll
86531     1.6133  tcp_transmit_skb

Of course, all CPUS fight on the dst_entry associated with 127.0.0.1 

Instead of first checking the refcount value, then decrement it,
we use atomic_dec_return() to help CPU to make the right memory transaction
(ie getting the cache line in exclusive mode)

dst_release() is now at the fifth position, and tbench a litle bit faster ;)

CPU: Core 2, speed 3000.1 MHz...