linux-kernel - Re: [PATCH 2/3] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if use_eager

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Date:	Thu, 29 Jan 2015 22:49:48 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] x86, fpu: unlazy_fpu: don't do __thread_fpu_end()
	if use_eager_fpu()

On 01/29, Rik van Riel wrote:
>
> On 01/29/2015 04:08 PM, Oleg Nesterov wrote:
> > unlazy_fpu()->__thread_fpu_end() doesn't look right if use_eager_fpu().
> > Unconditional __thread_fpu_end() is only correct if we know that this
> > thread can't return to user-mode and use FPU.
> >
> > Fortunately it has only 2 callers. fpu_copy() checks use_eager_fpu(),
> > and init_fpu(current) can be only called by the coredumping thread via
> > regset->get(). But it is exported to modules, and imo this should be
> > fixed anyway.
>
> What about xfpregs_set?
>
> It looks like that code copies an entire FPU state in
> from userspace, and expects the kernel to start using
> that new FPU state.
>
> This is called from the ptrace code.

Yes. But in this case tsk != current, and we ensure that __switch_to()
was finished. wait_task_inactive().

> When we switch to the traced task, the __thread_fpu_end()
> that was called from init_fpu() ensures that
> switch_fpu_begin() will actually load the new FPU state
> from memory into the registers, and we will not take
> the fpu_lazy_restore() branch.

No. in this case we rely on "tsk->thread.fpu.last_cpu = ~0" which disables
fpu_lazy_restore().

> What am I missing?

Or me ;)

Oleg.

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