[DSM-devel] On thread safety

Lars Wirzenius liw at iki.fi
Fri Aug 25 22:58:56 EEST 2006

There are a number of operations on lists, and perhaps other data
structures, that need to be protected by a mutex (or other construct).
It is probably not a good idea to put the mutex into the list nodes,
because that creates a lot of them and also creates a lot of intricate
locking requirements, and will therefore fail to be simple enough to
work correctly in all situations. I don't think we need to add the
mutexes right now, though.

The dsme_sock_getucred function needs to be fixed to not use a static
variable, but that should be easy.

Here's a list of static variables in the current code.


    static int as_lsock;
    static dsme_list_node_t as_clients;
    These are not necessarily a problem. There's probably not a need for
    multiple instances of the listening socket in one process, for
    example. It might be *prettier* to not have these two as
    process-global variables, but they shouldn't really hurt as it is.


    static struct { ... } logopt;
    static dsme_log_printer_t *printers[...];
    static int num_printers;
    These should be fine. There's no need to have different logging
    options for different threads. 
    The dsme_log_add_custom_method, dsme_log_open, and dsme_log_reopen
    functions are not re-entrant. That should be fixable via a simple
    mutex, if it is deemed necessary.


    static dsme_list_node_t modules;
    static dsme_list_node_t callbacks;
    static dsme_list_node_t message_queue;
    static const char *argv1;
    These should be good, except for the mutex protection that is


    static dsme_list_node_t connections;
    static dsme_list_node_t pseudo_connections;
    static int connectionlist_initialized;
    The connectionlist_initialized variable should go away;
    initialization should go into an initialization function that gets
    called once at the beginning of the process. Operations on the lists
    need mutex protection.
    function dsme_sock_getucred:
    static struct ucred pseudo_ucred;
    This is just wrong. The static variable needs to be replaced by
    something else.

    static dsme_sock_connection_t *pseudoconnection;
    OK, modulo a mutex.


    static dsme_list_node_t timers;
    OK, modulo a mutex.

    static module_t *my_handle;
    static dsmesock_connection_t *my_conn;
    static dsmesock_connection_t *connection;
    OK, modulo a mutex.

    static char lg_reboot_enabled;
    static dsme_module_t *my_handle;
    static dsme_sock_connection_t *my_conn;
    static dsme_list_node_t processes;
    static dsme_list_node_t uids;
    OK, modulo a mutex.

    static dsme_module_t *my_handle;
    static dsme_sock_connection_t *my_conn;
    static dsme_timer_t *timer;
    static int interval;
    static dsme_list_node_t processes;
    OK, modulo a mutex.


    static dsme_module_t *my_handle;
    static dsme_sock_connection_t *my_conn;

    OK, modulo a mutex.

Close your mind to stress and pain, hack till you're no longer sane.

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