linux-kernel - Re: [BUG 4.2-rc8] Interrupt occurs while apply

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Date:	Thu, 3 Sep 2015 08:53:44 +0100
From:	"Richard W.M. Jones" <rjones@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Chuck Ebbert <cebbert.lkml@...il.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [BUG 4.2-rc8] Interrupt occurs while apply_alternatives() is
 patching the handler

On Wed, Sep 02, 2015 at 08:05:12PM +0100, Richard W.M. Jones wrote:
> On Wed, Sep 02, 2015 at 11:11:55AM +0200, Thomas Gleixner wrote:
> > On Tue, 1 Sep 2015, Richard W.M. Jones wrote:
> > > On Sun, Aug 30, 2015 at 10:37:57PM -0400, Chuck Ebbert wrote:
> > > > This is from https://bugzilla.redhat.com/show_bug.cgi?id=1258223
> > > > 
> > > > [    0.036000] BUG: unable to handle kernel paging request at 55501e06
> > > [...]
> > > > [    0.036000]  [<c0409c80>] ? add_nops+0x90/0xa0
> > > > [    0.036000]  [<c040a054>] apply_alternatives+0x274/0x630
> > > > [    0.036000]  [<c07f1cf0>] ? wait_for_xmitr+0xa0/0xa0
> > > > [    0.036000]  [<c071a6fc>] ? sprintf+0x1c/0x20
> > > > [    0.036000]  [<c0aae480>] ? irq_entries_start+0x698/0x698
> > > > [    0.036000]  [<c071be4b>] ? memcpy+0xb/0x30
> > > > [    0.036000]  [<c07f3950>] ? serial8250_set_termios+0x20/0x20
> > > [...]
> > > > Interrupt 0x30 occurred while the alternatives code was replacing the
> > > > initial 0x90,0x90,0x90 NOPs (from the ASM_CLAC macro) with the optimized
> > > > version, 0x8d,0x76,0x00. Only the first byte has been replaced so far,
> > > > and it makes a mess out of the insn decoding.
> > 
> > apply_alternatives() has two ways to modify the code:
> > 
> > 1) text_poke_early()
> > 
> > 2) optimize_nops()
> > 
> > The former disables interrupts, the latter not. The patch below should
> > fix the issue.
> 
> It has gone through about 1100 iterations so far without hitting the
> bug.  I'll leave it running overnight.

That ran ~ 4000 iterations overnight, so it seems to work.  You can
add:

  Tested-by: Richard W.M. Jones <rjones@...hat.com>

Thanks,

Rich.

> Rich.
> 
> > Thanks,
> > 
> > 	tglx
> > 
> > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> > index c42827eb86cf..6a2f93e029f4 100644
> > --- a/arch/x86/kernel/alternative.c
> > +++ b/arch/x86/kernel/alternative.c
> > @@ -257,6 +257,9 @@ void __init arch_init_ideal_nops(void)
> >  /* Use this to add nops to a buffer, then text_poke the whole buffer. */
> >  static void __init_or_module add_nops(void *insns, unsigned int len)
> >  {
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> >  	while (len > 0) {
> >  		unsigned int noplen = len;
> >  		if (noplen > ASM_NOP_MAX)
> > @@ -265,6 +268,7 @@ static void __init_or_module add_nops(void *insns, unsigned int len)
> >  		insns += noplen;
> >  		len -= noplen;
> >  	}
> > +	local_irq_restore(flags);
> >  }
> >  
> >  extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
> > 
> > 
> > 
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
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-- 
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