On Tue, 9 Jan 2007 15:03:02 +0530
Srivatsa Vaddagiri <[email protected]> wrote:
> On Mon, Jan 08, 2007 at 09:26:56PM -0800, Andrew Morton wrote:
> > That's not correct. freeze_processes() will freeze *all* processes.
>
> I am not arguing whether all processes will be frozen. However my question was
> on the freeze point. Let me ask the question with an example:
>
> rtasd thread (arch/powerpc/platforms/pseries/rtasd.c) executes this simple
> loop:
>
>
> static int rtasd(void *unused)
> {
>
> i = first_cpu(cpu_online_map);
>
> while (1) {
>
> set_cpus_allowed(current, cpumask_of_cpu(i)); /* can block */
>
> /* we should now be running on cpu i */
>
> do_something_on_a_cpu(i);
>
> /* sleep for some time */
>
> i = next_cpu(cpu, cpu_online_map);
> }
>
> }
>
> This thread makes absolutely -no- calls to try_to_freeze() in its lifetime.
Looks like a bug to me. powerpc does appear to try to support the freezer.
> 1. Does this mean that the thread can't be frozen? (lets say that the
> thread's PF_NOFREEZE is not set)
yup. I'd expect the freeze_processes() call would fail if this thread is
running.
> AFAICS it can still be frozen by sending it a signal and have the signal
> delivery code call try_to_freeze() ..
kernel threads don't take signals in the same manner as userspace. A
kernel thread needs to explicitly poll, via
if (signal_pending(current))
do_something()
rtasd doesn't do that, and using signals in-kernel is considered lame.
> 2. If the thread can be frozen at any arbitrary point of its execution, then I
> dont see what prevents cpu_online_map from changing under the feet of rtasd
> thread,
It cannot.
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