Re: ptrace && fpu_lazy_restore

[Oleg Nesterov <oleg@redhat.com>]

On 04/14, Linus Torvalds wrote:
>
> So I actually think that I would prefer the patch that invalidates the
> FPU caches more aggressively. Sure, we don't really *need* to
> invalidate if we're just reading, but I'd almost prefer to just have
> it done once in "init_fpu()".

Agreed. I'll send your patch back to you tomorrow.

> The only case where we care about the FPU caches remaining is actually
> the nice normal "we just switched tasks through normal scheduling".

Yes. And there is another case when fpu_lazy_restore() returns the
false positive.

Suppose that fpu_owner_task exits on CPU_0, and then fork() reuses
its task_struct. The new child is still fpu_owner_task and this is
obviously wrong (unless of course another thread uses fpu).

Initially I thought this should be fixed too, but it seems that
"p->fpu_counter = 0" in copy_thread() saves us.

This looks a bit fragile... And could you confirm this is really
fine?


Btw, do we really need this "old->thread.fpu.last_cpu = ~0" in
the "else" branch of switch_fpu_prepare()? Just curious, I guees
this doesn't matter since we reset old->fpu_counter. But if we
can remove this line, then perhaps we can another optimization.

Oleg.