On Mon, Jul 22, 2013 at 6:41 PM, Rafael J. Wysocki <rjw@xxxxxxx> wrote:
> On Monday, July 22, 2013 05:42:49 PM Colin Cross wrote:
>> On Mon, Jul 22, 2013 at 5:32 PM, Linus Torvalds
>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> > On Mon, Jul 22, 2013 at 4:55 PM, Colin Cross <ccross@xxxxxxxxxxx> wrote:
>> >>
>> >> I think the right solution is to add a flag to the freezing task that
>> >> marks it unfreezable. I think PF_NOFREEZE would work, although it is
>> >> normally used on kernel threads, can you see if the attached patch
>> >> helps?
>> >
>> > Hmm. That does seem to be the right thing to do, but I wonder about
>> > the *other* callers of freeze_processes() IOW, kexec and friends.
>> >
>> > So maybe we should do this in {freeze|thaw}_processes() itself, and
>> > just make the rule be that the caller of freeze_processes() itself is
>> > obviously not frozen, and has to be the same one that then thaws
>> > things?
>> >
>> > Colin? Rafael? Comments?
>> >
>> > Linus
>>
>> I was worried about clearing the flag in thaw_processes(). If a
>> kernel thread with PF_NOFREEZE set ever called thaw_processes(), which
>> autosleep might do, it would clear the flag. Or if a different thread
>> called freeze_processes() and thaw_processes().
>
> Is that legitimate?
Nothing precludes it today, but I don't see any need for it. I'll add
a comment when I add the flag.
>> All the other callers besides the SNAPSHOT_FREEZE ioctl stay in the kernel
>> between freeze_processes() and thaw_processes(), which makes the fanout of
>> places that could call try_to_freeze() much more controllable.
>>
>> Using a new flag that operates like PF_NOFREEZE but doesn't conflict
>> with it, or a nofreeze_depth counter, would also work.
>
> Well, that would be robust enough. At least if the purpose of that new flag
> is clearly specified, people hopefully won't be tempted to optimize it away in
> the future.
>
> Thanks,
> Rafael
OK, I'll add a new flag.
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