linux-kernel - Re: PATCH? process_32.c:__switch_to() calls __math_state_restore() before updating current

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Message-ID: <20100208202652.GA27043@redhat.com>
Date:	Mon, 8 Feb 2010 21:26:52 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: PATCH? process_32.c:__switch_to() calls __math_state_restore()
	before updating current_task

On 02/08, Suresh Siddha wrote:
>
> On Fri, 2010-02-05 at 04:44 -0800, Oleg Nesterov wrote:
> > Could you please explain me another issue? I know nothing about fpu
> > handling, I am reading this code trying to understand the unrelated
> > bug with the old kernel.
> >
> > In short: why restore_i387_xstate() does init_fpu() when !used_math(),
> > can't (or shouldn't) it merely do set_used_math() ?
> >
> > 	restore_i387_xstate:
> >
> > 		if (!used_math()) {
> > 			err = init_fpu(tsk);
> > 			if (err)
> > 				return err;
> > 		}
> >
> > note that buf != NULL. This means that used_math() was true when
> > get_sigframe() was called, otherwise buf == sigcontext->fpstate
> > would be NULL, right?
>
> No. When the signal is delivered sigcontext->fpstate can be null and
> when we return from the signal handler, user can update the
> sigcontext->fpstate pointer

Yes, I already realized this, see my next emails.

> and the user expects the kernel to update
> the fpstate of the process accordingly.

but I didn't know it is "officially" allowed to populate .xstate
this way.


Thanks Suresh for your explanations.

Oleg.

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