Re: [PATCH] RFC: have tcp_recvmsg() check kthread_should_stop() and treat it as if it were signalled

["Satyam Sharma" <satyam.sharma@gmail.com>]

On 6/28/07, Oleg Nesterov <oleg@tv-sign.ru> wrote:
> (trimmed the cc: list)
> On 06/28, Satyam Sharma wrote:
> > On 6/27/07, Oleg Nesterov <oleg@tv-sign.ru> wrote:
> > >
> > >Contrary, I believe we should avoid signals when it
> > >comes to kernel threads.
> >
> > And I agree, but there's quite a subtle difference between signals being
> > used like they normally are, and this case here. Fact is, there is simply
> > no other way to break out of certain functions (if there was, I would've
> > preferred that myself).
> >
> > In fact, what I'm proposing is that kthreads should *not* be tinkering
> > with (flushing, handling, dequeueing, whatever) signals at all, because
> > like you mentioned, if they do that, it's possible that the TIF_SIGPENDING
> > could get lost.
>
> But we do have kthreads which call dequeue_signal(). And perhaps some
> kthread treats SIGKILL as "urgent exit with a lot of printks" while
> kthread_should_stop() means "exit gracefully".
> [..]
> I believe kthread_stop() should not send the signal. Just because it could
> be actually dequeued by kthread, and it may have some special meaning for
> this kthread.
> [...]
> Hmm... actually, such a change breaks the
>
>         while (signal_pending(current))
>                 dequeue_signal_and_so_something();
>
> loop, see jffs2_garbage_collect_thread() for example.
>
> In short, I think that kthread_stop() + TIF_SIGPENDING should be a special
> case, the signal should be sent explicitly before kthread_stop(), like
> cifs does.

The existence of such kthreads (that dabble in signals, although I guess
everybody here agrees kernel threads have no business dabbling in them)
is the *only* reason I didn't submit my proposal as an actual patch :-)

Perhaps "one day" we'll clean up all such cases, and fix up kthread
semantics to simply outlaw signal-handling in kernel threads (I just
can't convince myself there could be a good justifiable reason to do
that). When that's done, and seeing that kthreadd_setup() does
ignore_signals(), kthread_stop() could become simply force_sig() ...

> [...]
> This is what I can't understand completely. Why should we check SIGKILL
> or signal_pending() in addition to kthread_stop_info.k, what is the point?

... so kthread_stop_info will go away too.

> After all, the caller of kthread_stop(k) should know what and
> why k does.

One, the kthread code could change over time. Two, the solution
to the problem is un-intuitive for the user to do. The API should
know such cases, and handle them well too. The caller of
kthread_stop() would expect it to do the expected, after all (i.e.
stop the kthread :-)

> In any case, that kind of the changes can break things, just because
> this means API change.

Well, kthreads are a kernel-internal API anyway, I guess as long as we
fix all users, we're fine.

> > >I am talking about the case
> > >when wait_event_interruptible() should not fail (unless something bad
> > >happens) inside the "while (!kthread_should_stop())" loop.
> > >Note also that kthread could use TASK_INTERRUPTIBLE sleep
> > >[...] and because it knows that all signals are ignored.
> >
> > Ok, I think you're saying that if a kthread used wait_event_interruptible
> > (and was not expecting signals, because it ignores them), then bad
> > things (say exit in inconsistent state, etc) will happen if we break that
> > wait_event_interruptible unexpectedly.
>
> No. Of course, kthread should check the error and doesn't exit in
> inconsistent state.
>
> The question is: why should we break (say) tcp_recvmsg() inside
> "while (!kthread_should_stop())" loop if it is supposed to succeed
> unless something bad happens? (I mean, we may have a kthread which
> doesn't expect the failure unless something bad happens).

First, I don't quite understand this "not expecting failure" /
"something bad happens" bit at all.

Second, we *must* break that tcp_recvmsg() inside the kthread's
main loop, of course! We want it stopped, after all, and if we don't
make it "break" out of that function, the kthread _will_never_exit_.

[ What's worse, several kthreads are part of modules, and are
created during the module_init() and so the module_exit() function
needs to call kthread_stop() to clean it up. But the
wait_for_completion() in kthread_stop() will never unblock, and so
this would also mean the rmmod will _hang_ and module will never
get unloaded too. ]

> OK, let me give a silly example. The correctly written kthread should check
> the result of kmalloc(), yes? kthread(k) means that k should exit anyway, yes?
> So let's change kthread_stop(k) to also set TIF_MEMDIE, this means that
> __alloc_pages() will fail quickly when get_page_from_freelist() doesn't
> succeed, but won't start pageout() which may take a while. Please don't
> explain me why this suggestion is bad :), I am just trying to illustrate
> my point.

Your example / analogy is indeed *very* bad :-) Please note that this
whole thing is about functions that will _simply_*never*_exit_ever_
_unless_ given a signal. [ I think you've been misunderstanding my
proposal that I want to send a SIGKILL / signal to the kthread just
to ensure it gets stopped / killed "fast" or "asap" etc ... No! The only
reason I got into this was because kthread_stop() is an API that
fails to do what it should be doing if the corresponding kthread simply
happens to be executing certain functions in the kernel that will
*never* breakout unless given a signal. ]

But finally, I do agree that kthreads already exist out there who want
to dabble in signals and we can't break them, so perhaps the signal
method for kthread_stop() will have to wait for now. [ BTW even there
we're safe as long as we check kthread_stop() _before_ flushing or
dequeueing the signals, but then that'll be an ugly rule to try and
enforce, of course. ]

Satyam