1 files changed, 26 insertions, 20 deletions

diff --git a/openssl/crypto/bn/bn_mont.c b/openssl/crypto/bn/bn_mont.c
index 427b5cf4d..ee8532c7d 100644
--- a/openssl/crypto/bn/bn_mont.c
+++ b/openssl/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;
 	}