linux-kernel - Re: ptrace && fpu_lazy

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Message-ID: <20120415223808.GA26214@redhat.com>
Date:	Mon, 16 Apr 2012 00:38:08 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <chuckebbert.lk@...il.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: ptrace && fpu_lazy_restore

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.

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