Re: timer + fpu stuff locks my console race

From: Sergey Vlasov <vsu@altlinux.ru>
To: stian@nixia.no
Cc: Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: timer + fpu stuff locks my console race
Date: Sat, 12 Jun 2004 18:28:02 +0400	[thread overview]
Message-ID: <20040612142802.GB3396@sirius.home> (raw)
In-Reply-To: <1469.83.109.11.80.1087048662.squirrel@nepa.nlc.no>

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On Sat, Jun 12, 2004 at 03:57:42PM +0200, stian@nixia.no wrote:
> > --- linux-2.6.6/include/asm-i386/i387.h.fp-lockup	2004-05-10 06:33:06
> > +0400
> > +++ linux-2.6.6/include/asm-i386/i387.h	2004-06-12 17:25:56 +0400
> > @@ -51,7 +51,6 @@
> >  #define __clear_fpu( tsk )					\
> >  do {								\
> >  	if ((tsk)->thread_info->status & TS_USEDFPU) {		\
> > -		asm volatile("fwait");				\
> >  		(tsk)->thread_info->status &= ~TS_USEDFPU;	\
> >  		stts();						\
> >  	}							\
> 
> But what about task-switching and fpu-exceptions that comes in late? I
> know that the kernel does not use FPU in general, and the places it does,
> fsave, fwait and frstor embeddes it all in kernel-space.

Kernel code which uses FPU should call kernel_fpu_begin() before it
and kernel_fpu_end() after.  kernel_fpu_begin() is safe - it uses
fnsave or fxsave, both of which don't raise pending FPU exceptions.
Also fnsave performs implicit fninit, and fxsave is followed by
fnclex, which clears pending exceptions.

However, raid6_before_mmx() [drivers/md/raid6x86.h] seems to be buggy:

static inline void raid6_before_mmx(raid6_mmx_save_t *s)
{
	s->cr0 = raid6_get_fpu();
	asm volatile("fsave %0 ; fwait" : "=m" (s->fsave[0]));
}

fsave will raise pending exceptions (unlike fnsave).

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