Remove LLE read lock from IPv4 fast path.

LLE structure is mostly unchanged during its lifecycle.
To be more specific, there are 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 re-send arp request 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:
1) introduce special r_skip_req field which is read lockless by fast path,
  but updated under (new) req_mutex mutex. If this field is non-zero, then
  fast path will acquire lock and set it back to 0.
2) introduce simple state machine: incomplete->reachable<->verify->deleted.
  Before that we implicitely had incomplete->reachable->deleted state machine,
  with V_arpt_keep between "reachable" and "deleted". Verification was performed
  in runtime 5 seconds before V_arpt_keep expire.
  This is changed to "change state to verify 5 seconds before V_arpt_keep,
  set r_skip_req to non-zero value and check it every second". If the value
  is zero - then send arp verification probe.
These changes do not introduce any signifficant control plane overhead:
  typically lle callout timer would fire 1 time more each V_arpt_keep (1200s)
  for used lles and up to arp_maxtries (5) for dead lles.

As a result, all packets towards "reachable" lle are handled by fast path without
acquiring lle read lock.

Additional "req_mutex" is needed because callout / arpresolve_slow() or eventhandler
  might keep LLE lock for signifficant amount of time, which might not be feasible
  for fast path locking (e.g. having rmlock as ether AFDATA or lltable own lock).

Differential Revision:	https://reviews.freebsd.org/D3688

2 files changed, 16 insertions, 1 deletions

diff --git a/sys/net/if_llatbl.c b/sys/net/if_llatbl.c
index 0a89225..c7b1042 100644
--- a/sys/net/if_llatbl.c
+++ b/sys/net/if_llatbl.c
@@ -284,6 +284,7 @@ lltable_set_entry_addr(struct ifnet *ifp, struct llentry *lle,
 
 	bcopy(lladdr, &lle->ll_addr, ifp->if_addrlen);
 	lle->la_flags |= LLE_VALID;
+	lle->r_flags |= RLLE_VALID;
 }
 
 /*
@@ -640,6 +641,7 @@ lla_rt_output(struct rt_msghdr *rtm, struct rt_addrinfo *info)
 		if ((rtm->rtm_flags & RTF_ANNOUNCE))
 			lle->la_flags |= LLE_PUB;
 		lle->la_flags |= LLE_VALID;
+		lle->r_flags |= RLLE_VALID;
 		lle->la_expire = rtm->rtm_rmx.rmx_expire;
 
 		laflags = lle->la_flags;
diff --git a/sys/net/if_llatbl.h b/sys/net/if_llatbl.h
index 2009a7a..044959e 100644
--- a/sys/net/if_llatbl.h
+++ b/sys/net/if_llatbl.h
@@ -63,7 +63,8 @@ struct llentry {
 		uint16_t	mac16[3];
 		uint8_t		mac8[20];	/* IB needs 20 bytes. */
 	} ll_addr;
-	uint32_t		spare0;
+	uint16_t		r_flags;	/* LLE runtime flags */
+	uint16_t		r_skip_req;	/* feedback from fast path */
 	uint64_t		spare1;
 
 	struct lltable		 *lle_tbl;
@@ -83,6 +84,7 @@ struct llentry {
 	LIST_ENTRY(llentry)	lle_chain;	/* chain of deleted items */
 	struct callout		lle_timer;
 	struct rwlock		 lle_lock;
+	struct mtx		req_mtx;
 };
 
 #define	LLE_WLOCK(lle)		rw_wlock(&(lle)->lle_lock)
@@ -95,6 +97,12 @@ struct llentry {
 #define	LLE_LOCK_DESTROY(lle)	rw_destroy(&(lle)->lle_lock)
 #define	LLE_WLOCK_ASSERT(lle)	rw_assert(&(lle)->lle_lock, RA_WLOCKED)
 
+#define	LLE_REQ_INIT(lle)	mtx_init(&(lle)->req_mtx, "lle req", \
+	NULL, MTX_DEF)
+#define	LLE_REQ_DESTROY(lle)	mtx_destroy(&(lle)->req_mtx)
+#define	LLE_REQ_LOCK(lle)	mtx_lock(&(lle)->req_mtx)
+#define	LLE_REQ_UNLOCK(lle)	mtx_unlock(&(lle)->req_mtx)
+
 #define LLE_IS_VALID(lle)	(((lle) != NULL) && ((lle) != (void *)-1))
 
 #define	LLE_ADDREF(lle) do {					\
@@ -187,6 +195,11 @@ MALLOC_DECLARE(M_LLTABLE);
 #define	LLE_LINKED	0x0040	/* linked to lookup structure */
 /* LLE request flags */
 #define	LLE_EXCLUSIVE	0x2000	/* return lle xlocked  */
+#define	LLE_UNLOCKED	0x4000	/* return lle unlocked */
+
+/* LLE flags used by fastpath code */
+#define	RLLE_VALID	0x0001		/* entry is valid */
+#define	RLLE_IFADDR	LLE_IFADDR	/* entry is ifaddr */
 
 #define LLATBL_HASH(key, mask) \
 	(((((((key >> 8) ^ key) >> 8) ^ key) >> 8) ^ key) & mask)