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

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Date:	Mon, 08 Feb 2010 10:48:10 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Oleg Nesterov <oleg@...hat.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 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 and the user expects the kernel to update
the fpstate of the process accordingly.

> So, the task must have the valid ->thread.xstate, 

Need not be.

thanks,
suresh

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