RCU lock implementation

Introduce an RCU lock implementation as an alternative locking mechanism
to openssl.  The api is documented in the ossl_rcu.pod
file

Read side implementaiton is comparable to that of RWLOCKS:
ossl_rcu_read_lock(lock);
<
critical section in which data can be accessed via
ossl_derefrence
>
ossl_rcu_read_unlock(lock);

Write side implementation is:
ossl_rcu_write_lock(lock);
<
critical section in which data can be updated via
ossl_assign_pointer
and stale data can optionally be scheduled for removal
via ossl_rcu_call
>
ossl_rcu_write_unlock(lock);
...
ossl_synchronize_rcu(lock);

ossl_rcu_call fixup

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22729)

1 files changed, 31 insertions, 0 deletions

diff --git a/include/internal/rcu.h b/include/internal/rcu.h
new file mode 100644
index 0000000000..31a270222d
--- /dev/null
+++ b/include/internal/rcu.h
@@ -0,0 +1,31 @@
+/*
+ * Copyright 2023 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License 2.0 (the "License").  You may not use
+ * this file except in compliance with the License.  You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
+ */
+
+#ifndef OPENSSL_RCU_H
+# define OPENSSL_RCU_H
+# pragma once
+
+typedef void (*rcu_cb_fn)(void *data);
+
+typedef struct rcu_lock_st CRYPTO_RCU_LOCK;
+
+CRYPTO_RCU_LOCK *ossl_rcu_lock_new(int num_writers);
+void ossl_rcu_lock_free(CRYPTO_RCU_LOCK *lock);
+void ossl_rcu_read_lock(CRYPTO_RCU_LOCK *lock);
+void ossl_rcu_write_lock(CRYPTO_RCU_LOCK *lock);
+void ossl_rcu_write_unlock(CRYPTO_RCU_LOCK *lock);
+void ossl_rcu_read_unlock(CRYPTO_RCU_LOCK *lock);
+void ossl_synchronize_rcu(CRYPTO_RCU_LOCK *lock);
+int ossl_rcu_call(CRYPTO_RCU_LOCK *lock, rcu_cb_fn cb, void *data);
+void *ossl_rcu_uptr_deref(void **p);
+void ossl_rcu_assign_uptr(void **p, void **v);
+#define ossl_rcu_deref(p) ossl_rcu_uptr_deref((void **)p)
+#define ossl_rcu_assign_ptr(p,v) ossl_rcu_assign_uptr((void **)p, (void **)v)
+
+#endif