=pod

=head1 NAME

ossl_rcu_lock_new,
ossl_rcu_lock_free, ossl_rcu_read_lock,
ossl_rcu_read_unlock, ossl_rcu_write_lock,
ossl_rcu_write_unlock, ossl_synchronize_rcu,
ossl_rcu_call, ossl_rcu_deref,
ossl_rcu_assign_ptr, ossl_rcu_uptr_deref,
ossl_rcu_assign_uptr
- perform read-copy-update locking

=head1 SYNOPSIS

 CRYPTO_RCU_LOCK *ossl_rcu_lock_new(int num_writers, OSSL_LIB_CTX *ctx);
 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);
 void ossl_rcu_call(CRYPTO_RCU_LOCK *lock, rcu_cb_fn cb, void *data);
 void *ossl_rcu_deref(void **p);
 void ossl_rcu_uptr_deref(void **p);
 void ossl_rcu_assign_ptr(void **p, void **v);
 void ossl_rcu_assign_uptr(void **p, void **v);
 void ossl_rcu_lock_free(CRYPTO_RCU_LOCK *lock);

=head1 DESCRIPTION

OpenSSL can be safely used in multi-threaded applications provided that
support for the underlying OS threading API is built-in. Currently, OpenSSL
supports the pthread and Windows APIs. OpenSSL can also be built without
any multi-threading support, for example on platforms that don't provide
any threading support or that provide a threading API that is not yet
supported by OpenSSL.

In addition to more traditional Read/Write locks, OpenSSL provides
Read-Copy-Update (RCU) locks, which allow for always nonblocking read paths.

The following multi-threading functions are provided:

=over 2

=item *

ossl_rcu_assign_uptr() assigns the value pointed to by v to the
location pointed to by p.  This function should typically not be used, rely
instead on the ossl_rcu_assign_ptr() macro.

=item *

ossl_rcu_uptr_deref() returns the value stored at the
location pointed to by p. This function should typically not be used, rely
instead on the ossl_rcu_deref() macro.

=item *

ossl_rcu_assign_ptr() assigns the value pointed to by v to
location pointed to by p.

=item *

ossl_rcu_lock_new() allocates a new RCU lock.  The I<num_writers> param
indicates the number of write side threads which may execute
ossl_synchronize_rcu() in parallel.  The value must be at least 1, but may be
larger to obtain increased write side throughput at the cost of additional
internal memory usage.  A value of 1 is generally recommended. The I<ctx>
parameter references the library context in which the lock is allocated.

=item *

ossl_rcu_read_lock() acquires a read side hold on data protected by
the lock.

=item *

ossl_rcu_read_unlock() releases a read side hold on data protected by
the lock.

=item *

ossl_rcu_write_lock() acquires a write side hold on data protected by
the lock.  Note only one writer per lock is permitted, as with read/write locks.

=item *

ossl_rcu_write_unlock() releases a write side hold on data protected
by the lock.

=item *

ossl_synchronize_rcu() blocks the calling thread until all read side
holds on the lock have been released, guaranteeing that any old data updated by
the write side thread is safe to free.

=item *

ossl_rcu_call() enqueues a callback function to the lock, to be called
when the next synchronization completes.  Note: It is not guaranteed that the
thread which enqueued the callback will be the thread which executes the
callback

=item *

ossl_rcu_deref(p) atomically reads a pointer under an RCU locks
protection

=item *

ossl_rcu_assign_ptr(p,v) atomically writes to a pointer under an
RCU locks protection

=item *

ossl_rcu_lock_free() frees an allocated RCU lock

=back

=head1 RETURN VALUES

ossl_rcu_lock_new() returns a pointer to a newly created RCU lock structure.

ossl_rcu_deref() and ossl_rcu_uptr_deref() return the value pointed
to by the passed in value v.

All other functions return no value.

=head1 EXAMPLES

You can find out if OpenSSL was configured with thread support:

 #include <openssl/opensslconf.h>
 #if defined(OPENSSL_THREADS)
     /* thread support enabled */
 #else
     /* no thread support */
 #endif

This example safely initializes and uses a lock.

 #include "internal/rcu.h"

 struct foo {
    int aval;
    char *name;
 };

 static CRYPTO_ONCE once = CRYPTO_ONCE_STATIC_INIT;
 static CRYPTO_RCU_LOCK *lock;
 static struct foo *fooptr = NULL;

 static void myinit(void)
 {
     lock = ossl_rcu_lock_new(1);
 }

 static int initlock(void)
 {
     if (!RUN_ONCE(&once, myinit) || lock == NULL)
         return 0;
     return 1;
 }

 static void writer_thread()
 {
    struct foo *newfoo;
    struct foo *oldfoo;

    initlock();

    /*
     * update steps in an rcu model
     */

    /*
     * 1) create a new shared object
     */
    newfoo = OPENSSL_zalloc(sizeof(struct foo));

    /*
     * acquire the write side lock
     */
    ossl_rcu_write_lock(lock);

    /*
     * 2) read the old pointer
     */
    oldfoo = ossl_rcu_deref(&fooptr);

    /*
     * 3) Copy the old pointer to the new object, and
     *    make any needed adjustments
     */
    memcpy(newfoo, oldfoo, sizeof(struct foo));
    newfoo->aval++;

    /*
     * 4) Update the shared pointer to the new value
     */
    ossl_rcu_assign_ptr(&fooptr, &newfoo);

    /*
     * 5) Release the write side lock
     */
    ossl_rcu_write_unlock(lock);

    /*
     * 6) wait for any read side holds on the old data
     *    to be released
     */
    ossl_synchronize_rcu(lock);

    /*
     * 7) free the old pointer, now that there are no
     *    further readers
     */
    OPENSSL_free(oldfoo);
 }

 static void reader_thread()
 {
    struct foo *myfoo = NULL;
    int a;
    /*
     * 1) Acquire a read side hold on the shared data
     */
    ossl_rcu_read_lock(lock);

    /*
     * 2) Access the shared data pointer
     */
    myfoo = ossl_rcu_deref(&fooptr);

    /*
     * 3) Read the data from the pointer
     */
    a = myfoo->aval;

    /*
     * 4) Indicate our hold on the shared data is complete
     */
    ossl_rcu_read_unlock(lock);
 }

=head1 SEE ALSO

L<crypto(7)>, L<openssl-threads(7)>.

=head1 COPYRIGHT

Copyright 2023-2024 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
L<https://www.openssl.org/source/license.html>.

=cut