Re: re. Spurious wakeup on a newly created kthread

Peter Zijlstra
2022-06-27T07:20:11Z

On Mon, Jun 27, 2022 at 12:01:04AM +0000, Wedson Almeida Filho wrote:
> On Sat, Jun 24, 2022 at 07:19:27PM -0500, Eric W. Biederman wrote:
> >
> > Further it is necessary for Peter Zijlstra's rewrite of the kernel
> > freezer. As anything that isn't a special stop state (which
> > TASK_UNINTERRUPTIBLE is not) will receive a spurious wake up on when
> > thawed out.
>
> Do you know if the current (i.e., prior to the rewrite) kernel freezer
> also sends spurious wakeups when thawing tasks?

Current freezer can thaw at random points in time, even before SMP
bringup if you're unlucky. And yes, I think it can induce 'spurious'
wakeups as well.

But really; like Linus already said upsteam, every wait loop *MUST*
already be able to deal with spurious wakeups. This is why pretty much
every wait primitive we have looks like:

for (;;) {
set_current_state(state);
if (cond)
break;
schedule();
}
__set_current_state(RUNNING);

Which is immune to random wake-ups since it need @cond to make progress.
*NEVER* rely on just the wakeup itself for progress, that's buggy as
heck in lots of ways.

There are a few exceptions, but they all require special wait states and
much carefulness.