Re: [Suspend-devel] [RFC][PATCH -mm 1/5] PM: Make freeze_processes SMP-safe

Rafael J. Wysocki
2006-11-30T21:55:52Z

On Thursday, 30 November 2006 16:07, Rafael J. Wysocki wrote:
> On Thursday, 30 November 2006 01:21, Rafael J. Wysocki wrote:
> > On Thursday, 30 November 2006 00:55, Pavel Machek wrote:
> > > Hi!
> > >
> > > > > > I do not like the counting idea; it should be simpler to just check if
> > > > > > all the processes are still stopped.
> > > > >
> > > > > I thought about that but didn't invent anything reasonable enough.
> > > > >
> > > > > > But I'm not sure if this is enough. What if signal is being delivered
> > > > > > on another CPU while freezing, still being delivered while this second
> > > > > > check runs, and then SIGCONT is delivered?
> > > > >
> > > > > Hm, is this possible in practice? I mean, if todo is 0 and nr_stopped doesn't
> > > > > change, then there are no processes that can send the SIGCONT (unless someone
> > > > > creates a kernel thread with PF_NOFREEZE that will do just that).
> > > > >
> > > > > Anyway, for now I've no idea how to fix this properly. Will think about it
> > > > > tomorrow.
> > > >
> > > > As far as this particular problem is concerned, I think there are two possible
> > > > solutions.
> > > >
> > > > One of them would be do disable the delivery of continuation signals before
> > > > we start freezing processes, but I don't know how to do this exactly so that
> > > > it's not racy. Also it would be quite intrusive.
> > > >
> > > > The other one may be something along with the lines of the appended patch.
> > >
> > > There has to be a better solution. Stopped tasks are suspended
> > > somewhere in kernel, right? One try_to_freeze() and problem should be
> > > solved, in regular way, and without tricks...?
> >
> > Why? _This_ is a regular way, IMHO.
> >
> > The problem is that stopped tasks aren't actually running (obviously) so they
> > _can't_ execute try_to_freeze() until someone sends them a signal. However,
> > once they actually have received the signal, we want them to freeze, so we
> > must tell them to do so. Still, if they don't receive the signal, we want them
> > to stay stopped (IOW, the freezer by itself should not wake them up).
>
> <--snip-->
>
> In fact, I really mean that if we want a process to go to the refrigerator, we
> have to set PF_FREEZE for it (otherwise try_to_freeze() won't do anytning).
> Thus because we want stopped processes to go to the refrigerator once they
> have received the continuation signal, we have to set PF_FREEZE for them,
> so we should call either freeze_process() or just freeze() for them.
>
> Now once we have set PF_FREEZE for a stopped process, we shouldn't count
> it as freezeable any more, because we can't do anything more with it.
> Moreover, if the process hasn't received the continuation signal before we
> call freeze_processes(), PF_FREEZE set will still be set for it, so we have to
> clear it (otherwise the process would go to the refrigerator as soon as it
> receives the continuation signal).
>
> Now the question remains if we should call the entire freeze_process() or just
> freeze() for stopped tasks and I think it really doesn't matter. Still, since we
> call recalc_sigpending() in the refrigerator, I think it's reasonable to use
> freeze_process() in this case (less lines of code).
>
> Additionally, we can move the try_to_freeze() in get_signal_to_deliver() so
> that processes receiving continuation signals are frozen immediately rather
> than some time later, but this doesn't really change the rest of the patch
> (which follows - untested for now, but I'll test it later today).

Now tested and it doesn't break anything, at least.

Greetings,
Rafael