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Date: Thu, 3 Sep 2015 10:50:46 +0200
From: Borislav Petkov <bp@alien8.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Richard W.M. Jones" <rjones@redhat.com>,
        Chuck Ebbert <cebbert.lkml@gmail.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [BUG 4.2-rc8] Interrupt occurs while apply_alternatives() is
 patching the handler
Message-ID: <20150903085046.GA19397@nazgul.tnic>
References: <20150830223757.6e4c5c02@as>
 <20150901062022.GA19002@redhat.com>
 <alpine.DEB.2.11.1509021108570.15006@nanos>
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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.
> 
> 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) {

I guess you want to optimize the len==0 case to not disable interrupts
needlessly:

	if (!len)
		return;

	local_irq_save(flags);
	while (len > 0)
		...

Other than that, good catch!

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--