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LOCKING RULES AND INVARIANTS:
invariants:
meaning of js_state are the following:
JS_STATE_UNDEFINED means no player thread.
JS_STATE_BUFFERING means thread is waiting on the condition variable.
JS_STATE_INITIALIZING means thread was just started, but did not start
waiting on the condition variable yet.
JS_STATE_TRANSITIONING means player thread is about to close the
connection and accessing control pipe is unsafe
(or control == 0).
JS_STATE_PLAYING etc have the obvious meanings.
Locking Rules:
(0) Rule: when player thread is dead (i.e. in shut after the
pthread_join), no locking is needed (and no locking rules apply)..
(1) Rule: all read or write access to instance->js_state (in all threads) must
be protected by control_mutex.
(2) Rule: all read and write access to the playlist (i.e. td->list) must
be protected by playlist_mutex.
(3) Rule: No thread may block with control_mutex held.
(4) Rule: If one needs to acquire both playlist_mutex and control_mutex,
playlist_mutex must be acquired first. Doing it in the other order
will deadlock.
(5) Rule: all access to instance->control (from both threads) must be protected
by control_mutex.
(6) Exception to last rule: when js_state == JS_STATE_TRANSITIONING
player thread does not need locks for instance->control,
but access by main thread is prohibuted.
(7) Rule: In the player thread, any code which looks like
pthread_mutex_lock(&some_lock);
.... do stuff ....
pthread_mutex_unlock(&some_lock);
where the ... do stuff ... includes a cancellation point,
must be protected by pthread_cleanup_push(pthread_mutex_unlock, some_lock)
and pthread_cleanup_pop(0)
Recall that pthread_test_cancel(), pthread_cond_wait() and any
blocking system call/libc function is a cancellation point.
(8) Rule: instance->player is owned by the player thread. The only time
main thread can access it is when the player thread is already
dead (e.g. in shut after the pthread_join).
(9) Corollary to above rules:
All calls to sendCommand have to protected by control_mutex. Also
in player thread, calls to sendCommand have to be
protected by cleanup_push/cleanup_pop as well.
(10) Rule: Player thread may be started if there isn't one currently
(i.e. js_state == JS_STATE_UNDEFINED). To start player thread, must do:
pthread_lock(&control_mutex);
pthread_create(....)
js_state = JS_STATE_INITIALIZING;
pthread_unlock(&control_mutex);
(11) Rule: to signal player thread, must use the signalPlayerThread()
function.
(12) Rule: to kill player thread, must do:
pthread_cancel(&player_thread);
pthread_join(&player_thread);
js_state = JS_STATE_UNDEFINED;
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