Add a warning about locking in the child provider callback docs
The child provider callbacks can hold the store lock. In order to avoid
deadlocks we require that the callback implementations don't themselves
call functions that may aquire those locks.
@@ -123,7 +123,7 @@ provider-base
All "functions" mentioned here are passed as function pointers between
F<libcrypto> and the provider in B<OSSL_DISPATCH> arrays, in the call
of the provider initialization function. See L<provider(7)/Provider>
-for a description of the initialization function.
+for a description of the initialization function. They are known as "upcalls".
All these "functions" have a corresponding function type definition
named B<OSSL_FUNC_{name}_fn>, and a helper function to retrieve the
@@ -328,7 +328,9 @@ provider_register_child_cb() registers callbacks for being informed about the
loading and unloading of providers in the application's library context.
I<handle> is this provider's handle and I<cbdata> is this provider's data
that will be passed back to the callbacks. It returns 1 on success or 0
-otherwise.
+otherwise. These callbacks may be called while holding locks in libcrypto. In
+order to avoid deadlocks the callback implementation must not be long running
+and must not call other OpenSSL API functions or upcalls.
I<create_cb> is a callback that will be called when a new provider is loaded
into the application's library context. It is also called for any providers that
