On Sat, 23 Feb 2008, Oleg Nesterov wrote:
> (s/mm-commits/lkml, cc Steven and Linus).
Thanks,
>
> On 02/22, Andrew Morton wrote:
> >
> > From: Dmitry Adamushko <[EMAIL PROTECTED]>
> >
> > We must ensure that kthread_stop_info.k has been updated before kthread's
> > wakeup. This is required to properly support the use of
> > kthread_should_stop()
> > in the main loop of kthread.
> >
> > wake_up_process() doesn't imply a full memory barrier,
> > so we add an explicit one.
>
> I tried to raise the similar issue twice without success.
>
> I think we should fix wake_up_process() instead.
At first I was thinking that this may be too much on such an highly used
API. But you may be right. I did a quick seach on who uses
wake_up_process. I randomly picked one. ptrace.
I think we have a bug there. And this was just by randomly looking at
it.
In kernel/ptrace.c: ptrace_resume
child->exit_code = data;
wake_up_process(child);
Again, there's no guarantee that exit_code will equal data when the child
wakes up.
And in something like do_syscall_trace, we have
ptrace_notify(SIGTRAP | ((current->ptrace & PT_TRACESYSGOOD) ?
0x80:0));
/*
* this isn't the same as continuing with a signal, but it will do
* for normal use. strace only continues with a signal if the
* stopping signal is not SIGTRAP. -brl
*/
if (current->exit_code) {
ptrace_notify eventually calls ptrace_stop which schedules. This is what
gets woken up. There is a possible chance that current->exit_code will not
equal data in the ptrace_resume code. That is, if wake_up_process implies
no barrier.
I would recommend having a wake_up_process version that does not imply a
barrier, so we can keep straight forward wakeups fast and not
unnecessarily add barriers.
Good catch Oleg!
-- Steve
>
> Please look at http://marc.info/?l=linux-kernel&m=118503598307267
> and at this thread: http://marc.info/?t=116275561700001
>
> In short: wake_up_process() doesn't imply mb(), this means that _in theory_
> the commonly used code like
>
> set_current_state(TASK_INTERRUPTIBLE);
> if (CONDITION)
> return;
> schedule();
>
> is racy wrt
>
> CONDITION = 1;
> wake_up_process(p);
>
> I'll be happy to be wrong though, please correct me.
>
> Oleg.
>
>
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