Re: [CRYPTO]: Only reschedule if !in_atomic()

[Andrew Morton <akpm@osdl.org>]

Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
>
> On Mon, 2005-05-23 at 16:28 -0700, Andrew Morton wrote:
> 
> > This code can cause deadlocks on CONFIG_SMP && !CONFIG_PREEMPT kernels.
> > 
> > Please see http://lkml.org/lkml/2005/3/10/358
> > 
> > You (the programmer) *have* to know what context you're running in before
> > doing a voluntary yield.  There is simply no way to work this out at
> > runtime.
> 
> Hrm... Linus just merged it though...
> 

The old version was:

	if (!in_softirq())
		cond_resched();

Which I guess is OK if the programmer knows that this code is only ever called

a) from softirq context or

b) from process context with no locks/smp_processor_id/etc held.

The new version is:

	if (!in_atomic())
		cond_resched();

which happens to still be correct as long as a) and b) still hold, which I
assume they do.

Both versions are deadlocky if b) is violated.

So.  It sucks before and it sucks after, but we might not be deadlocky. 
Problem is, !CONFIG_PREEMPT also disable the beancounting which
might_sleep() depends upon, so it's harder to tell whether all callers are
correct.