Commit 71e8b67d authored by Alexander Duyck's avatar Alexander Duyck Committed by David S. Miller
Browse files

fib_trie: Update last spot w/ idx >> n->bits code and explanation 



This change updates the fib_table_lookup function so that it is in sync
with the fib_find_node function in terms of the explanation for the index
check based on the bits value.

I have also updated it from doing a mask to just doing a compare as I have
found that seems to provide more options to the compiler as I have seen it
turn this into a shift of the value and test under some circumstances.

In addition I addressed one minor issue in which we kept computing the key
^ n->key when checking the fib aliases.  I pulled the xor out of the loop
in order to reduce the number of memory reads in the lookup.  As a result
we should save a couple cycles since the xor is only done once much earlier
in the lookup.

Signed-off-by: default avatarAlexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent a7e53531

net/ipv4/fib_trie.c

+13 −5
Original line number Diff line number Diff line
@@ -1201,6 +1201,7 @@ int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp, 
	const t_key key = ntohl(flp->daddr);

	struct tnode *n, *pn;

	struct fib_alias *fa;

	unsigned long index;

	t_key cindex;



	n = rcu_dereference(t->trie);
@@ -1216,19 +1217,23 @@ int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp, 


	/* Step 1: Travel to the longest prefix match in the trie */

	for (;;) {

		unsigned long index = get_index(key, n);

		index = get_index(key, n);



		/* This bit of code is a bit tricky but it combines multiple

		 * checks into a single check.  The prefix consists of the

		 * prefix plus zeros for the "bits" in the prefix. The index

		 * is the difference between the key and this value.  From

		 * this we can actually derive several pieces of data.

		 *   if (index & (~0ul << bits))

		 *   if (index >= (1ul << bits))

		 *     we have a mismatch in skip bits and failed

		 *   else

		 *     we know the value is cindex

		 *

		 * This check is safe even if bits == KEYLENGTH due to the

		 * fact that we can only allocate a node with 32 bits if a

		 * long is greater than 32 bits.

		 */

		if (index & (~0ul << n->bits))

		if (index >= (1ul << n->bits))

			break;



		/* we have found a leaf. Prefixes have already been compared */
@@ -1302,12 +1307,15 @@ backtrace: 
	}



found:

	/* this line carries forward the xor from earlier in the function */

	index = key ^ n->key;



	/* Step 3: Process the leaf, if that fails fall back to backtracing */

	hlist_for_each_entry_rcu(fa, &n->leaf, fa_list) {

		struct fib_info *fi = fa->fa_info;

		int nhsel, err;



		if (((key ^ n->key) >= (1ul << fa->fa_slen)) &&

		if ((index >= (1ul << fa->fa_slen)) &&

		    ((BITS_PER_LONG > KEYLENGTH) || (fa->fa_slen != KEYLENGTH)))

			continue;

		if (fa->fa_tos && fa->fa_tos != flp->flowi4_tos)

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