MFC r291301:

The r241129 description was wrong that the scenario is possible
only for read locks on pcbs. The same race can happen with write
lock semantics as well.

The race scenario:

• Two threads (1 and 2) locate pcb with writer semantics (INPLOOKUP_WLOCKPCB) and do in_pcbref() on it.
• 1 and 2 both drop the inp hash lock.
• Another thread (3) grabs the inp hash lock. Then it runs in_pcbfree(), which wlocks the pcb. They must happen faster than 1 or 2 come INP_WLOCK()!
• 1 and 2 congest in INP_WLOCK().
• 3 does in_pcbremlists(), drops hash lock, and runs in_pcbrele_wlocked(), which doesn't free the pcb due to two references on it. Then it unlocks the pcb.
• 1 (or 2) gets wlock on the pcb, runs in_pcbrele_wlocked(), which doesn't report inp as freed, due to 2 (or 1) still helding extra reference on it. The thread tries to do smth with a disconnected pcb and crashes.
  
  Submitted by: emeric.poupon@stormshield.eu Reviewed by: glebius@ Sponsored by: Stormshield Tested by: Cassiano Peixoto, Stormshield