linux-kernel - Re: [BUG REPORT] Soft Lockup in smp_call_function

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Message-ID: <CALCETrXFfCqCEtghmXARH-2N9EsuzkHAYX-qzeF8qSeqvw4wrA@mail.gmail.com>
Date:	Sat, 30 Jan 2016 08:53:27 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Jeff Merkey <linux.mdb@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Andrew Lutomirski <luto@...nel.org>
Subject: Re: [BUG REPORT] Soft Lockup in smp_call_function_single+0xD8

On Sat, Jan 30, 2016 at 12:41 AM, Jeff Merkey <linux.mdb@...il.com> wrote:
> Here is an MDB debugger trace of the code in question.  please note
> that the flags being compared don't match what's in r11 and the
> comparison bits are wrong.
>
> (3)>
>
> Break at 0xFFFFFFFF81680022 due to - Proceed (single step)
> RAX: 0000000000000080 RBX: 0000000000000002 RCX: 00007FC9877F2A30
> RDX: 0000000000000000 RSI: FFFF8800BFD9BC00 RDI: FFFF88011FCD6C80
> RSP: FFFF8800CD6C7F58 RBP: 00007FC988119000  R8: FFFF8800CD6C4000
>  R9: 0000017C85499D0E R10: FFFF8800C17BB8F0 R11: 0000000000000246  << WRONG!!!
> R12: 00007FC987AC6400 R13: 0000000000000002 R14: 0000000000000001
> R15: 0000000000000000 CS: 0010 DS: 0000 ES: 0000 FS: 0000 GS: 0000 SS: 0018
>  IP: FFFFFFFF81680022 FLAGS: 0000000000000146  (PF ZF TF) << real flags
> 0xffffffff81680022 49F7C300010100  test   r11,0x10100   < comparison
> bits correct r11 is WRONG!!!
> (3)>

I have no idea what bug you're talking about, and I have no idea how
this code could cause a soft lockup in smp_call_function_single (at
worst it could potentially enter userspace with invalid state, this
alternating between user and kernel without making progress in user
mode).

And the HW flags register has no particular reason to match r11 or, in
fact, anything saved in pt_regs at all.

--Andy