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Date: Wed, 2 Sep 2015 11:11:55 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: "Richard W.M. Jones" <rjones@redhat.com>
cc: 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
In-Reply-To: <20150901062022.GA19002@redhat.com>
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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) {
 		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[];