Remove LLE read lock from IPv6 fast path.

LLE structure is mostly unchanged during its lifecycle: there are only 2
things relevant for fast path lookup code:
1) link-level address change. Since r286722, these updates are performed
  under AFDATA WLOCK.
2) Some sort of feedback indicating that this particular entry is used so
  we send NS to perform reachability verification instead of expiring entry.
  The only signal that is needed from fast path is something like binary
  yes/no.
The latter is solved by the following changes:

Special r_skip_req (introduced in D3688) value is used for fast path feedback.
  It is read lockless by fast path, but updated under req_mutex mutex. If this
  field is non-zero, then fast path will acquire lock and set it back to 0.

After transitioning to STALE state, callout timer is armed to run each
  V_nd6_delay seconds to make sure that if packet was transmitted at the start
  of given interval, we would be able to switch to PROBE state in V_nd6_delay
  seconds as user expects.
(in STALE state) timer is rescheduled until original V_nd6_gctimer expires
  keeping lle in STALE state (remaining timer value stored in lle_remtime).
(in STALE state) timer is rescheduled if packet was transmitted less that
  V_nd6_delay seconds ago to make sure we transition to PROBE state exactly
  after V_n6_delay seconds.

As a result, all packets towards lle in REACHABLE/STALE/PROBE states are handled
  by fast path without acquiring lle read lock.

Differential Revision:		https://reviews.freebsd.org/D3780

1 files changed, 41 insertions, 0 deletions

diff --git a/sys/net/if_llatbl.c b/sys/net/if_llatbl.c
index c7b1042..f2749e5 100644
--- a/sys/net/if_llatbl.c
+++ b/sys/net/if_llatbl.c
@@ -288,6 +288,47 @@ lltable_set_entry_addr(struct ifnet *ifp, struct llentry *lle,
 }
 
 /*
+ * Tries to update @lle link-level address.
+ * Since update requires AFDATA WLOCK, function
+ * drops @lle lock, acquires AFDATA lock and then acquires
+ * @lle lock to maintain lock order.
+ *
+ * Returns 1 on success.
+ */
+int
+lltable_try_set_entry_addr(struct ifnet *ifp, struct llentry *lle,
+    const char *lladdr)
+{
+
+	/* Perform real LLE update */
+	/* use afdata WLOCK to update fields */
+	LLE_WLOCK_ASSERT(lle);
+	LLE_ADDREF(lle);
+	LLE_WUNLOCK(lle);
+	IF_AFDATA_WLOCK(ifp);
+	LLE_WLOCK(lle);
+
+	/*
+	 * Since we droppped LLE lock, other thread might have deleted
+	 * this lle. Check and return
+	 */
+	if ((lle->la_flags & LLE_DELETED) != 0) {
+		IF_AFDATA_WUNLOCK(ifp);
+		LLE_FREE_LOCKED(lle);
+		return (0);
+	}
+
+	/* Update data */
+	lltable_set_entry_addr(ifp, lle, lladdr);
+
+	IF_AFDATA_WUNLOCK(ifp);
+
+	LLE_REMREF(lle);
+
+	return (1);
+}
+
+/*
  *
  * Performes generic cleanup routines and frees lle.
  *