Re: [RFC][PATCH -mm 2/6] Freezer: Do not send signals to kernel threads

Oleg Nesterov
2007-07-10T15:00:50Z

On 07/09, Rafael J. Wysocki wrote:
>
> Commit b74d0deb968e1f85942f17080eace015ce3c332c has changed
> recalc_sigpending_tsk() so that it doesn't clear TIF_SIGPENDING. For this
> reason, the freezer should not send fake signals to kernel threads any more,
> since otherwise some of them may run with TIF_SIGPENDING set forever if the
> freezing of kernel threads fails.

I personally think it is very good to get rid of signals to kthread, regardless
of changed behaviour of recalc_sigpending_tsk().

> +static int freeze_user_process(struct task_struct *p)
> +{
> + int ret = 1;
> +
> + task_lock(p);
> + if (has_mm(p)) {
> + if (freezing(p)) {
> + if (!signal_pending(p))
> + fake_signal_wake_up(p, 0);
> + else
> + wake_up_state(p, TASK_INTERRUPTIBLE);

Why do we need the "else" branch? It is already a bug if the task sleeps
in TASK_INTERRUPTIBLE state but has signal_pending().

The same for freeze_task(). Actually, they look very similar. Imho, it would
be better to have a single function with a "user_space_only" parameter.

> @@ -99,7 +99,6 @@ static inline int has_pending_signals(si
> static int recalc_sigpending_tsk(struct task_struct *t)
> {
> if (t->signal->group_stop_count > 0 ||
> - (freezing(t)) ||
> PENDING(&t->pending, &t->blocked) ||
> PENDING(&t->signal->shared_pending, &t->blocked)) {
> set_tsk_thread_flag(t, TIF_SIGPENDING);

This is important (and imho good) change, but changelog says nothing about it.

I guess this should works because freeze_user_process/freeze_task repeatedly
"re-send" the signal in a loop when TIF_SIGPENDING is cleared, yes?

> +#define wait_event_freezable(wq, condition) \
> +({ \
> + int __ret; \
> + do { \
> + __ret = wait_event_interruptible(wq, \
> + (condition) || freezing(current)); \
> + } while (try_to_freeze()); \
> + __ret; \
> +})

I don't think this is right.

wait_event_freezable() should return success _only_ if "condition" == true.
What if TIF_FREEZE was cleared between freezing() and try_to_freeze() ?

> --- linux-2.6.22-rc6-mm1.orig/drivers/input/gameport/gameport.c
> +++ linux-2.6.22-rc6-mm1/drivers/input/gameport/gameport.c
> @@ -448,9 +448,8 @@ static int gameport_thread(void *nothing
> set_freezable();
> do {
> gameport_handle_event();
> - wait_event_interruptible(gameport_wait,
> + wait_event_freezable(gameport_wait,
> kthread_should_stop() || !list_empty(&gameport_event_list));
> - try_to_freeze();
> } while (!kthread_should_stop());

Isn't it better to break this patch into 2 separate ones? The first adds
wait_event_freezable() and "fixes" gameport_thread() and friends in advance,
the second deals with TIF_SIGPENDING. (please ignore if this is not convenient).

Oleg.