bignum: allow concurrent BN_MONT_CTX_set_locked()

The lazy-initialisation of BN_MONT_CTX was serialising all threads, as
noted by Daniel Sands and co at Sandia. This was to handle the case that
2 or more threads race to lazy-init the same context, but stunted all
scalability in the case where 2 or more threads are doing unrelated
things! We favour the latter case by punishing the former. The init work
gets done by each thread that finds the context to be uninitialised, and
we then lock the "set" logic after that work is done - the winning
thread's work gets used, the losing threads throw away what they've done.

Signed-off-by: Geoff Thorpe <geoff@openssl.org>

1 files changed, 26 insertions, 20 deletions

diff --git a/crypto/bn/bn_mont.c b/crypto/bn/bn_mont.c
index 427b5cf4df..ee8532c7dc 100644
--- a/crypto/bn/bn_mont.c
+++ b/crypto/bn/bn_mont.c
@@ -478,32 +478,38 @@ BN_MONT_CTX *BN_MONT_CTX_copy(BN_MONT_CTX *to, BN_MONT_CTX *from)
 BN_MONT_CTX *BN_MONT_CTX_set_locked(BN_MONT_CTX **pmont, int lock,
 					const BIGNUM *mod, BN_CTX *ctx)
 	{
-	int got_write_lock = 0;
 	BN_MONT_CTX *ret;
 
 	CRYPTO_r_lock(lock);
-	if (!*pmont)
+	ret = *pmont;
+	CRYPTO_r_unlock(lock);
+	if (ret)
+		return ret;
+
+	/* We don't want to serialise globally while doing our lazy-init math in
+	 * BN_MONT_CTX_set. That punishes threads that are doing independent
+	 * things. Instead, punish the case where more than one thread tries to
+	 * lazy-init the same 'pmont', by having each do the lazy-init math work
+	 * independently and only use the one from the thread that wins the race
+	 * (the losers throw away the work they've done). */
+	ret = BN_MONT_CTX_new();
+	if (!ret)
+		return NULL;
+	if (!BN_MONT_CTX_set(ret, mod, ctx))
 		{
-		CRYPTO_r_unlock(lock);
-		CRYPTO_w_lock(lock);
-		got_write_lock = 1;
+		BN_MONT_CTX_free(ret);
+		return NULL;
+		}
 
-		if (!*pmont)
-			{
-			ret = BN_MONT_CTX_new();
-			if (ret && !BN_MONT_CTX_set(ret, mod, ctx))
-				BN_MONT_CTX_free(ret);
-			else
-				*pmont = ret;
-			}
+	/* The locked compare-and-set, after the local work is done. */
+	CRYPTO_w_lock(lock);
+	if (*pmont)
+		{
+		BN_MONT_CTX_free(ret);
+		ret = *pmont;
 		}
-	
-	ret = *pmont;
-	
-	if (got_write_lock)
-		CRYPTO_w_unlock(lock);
 	else
-		CRYPTO_r_unlock(lock);
-		
+		*pmont = ret;
+	CRYPTO_w_unlock(lock);
 	return ret;
 	}