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Date:	Thu, 24 Jul 2014 22:36:54 -0400
From:	Nick Krause <xerofoify@...il.com>
To:	Michel Dänzer <michel@...nzer.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jakub Jelinek <jakub@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Debian GCC Maintainers <debian-gcc@...ts.debian.org>,
	Debian Kernel Team <debian-kernel@...ts.debian.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On Thu, Jul 24, 2014 at 9:25 PM, Michel Dänzer <michel@...nzer.net> wrote:
> [ Adding the Debian kernel and gcc teams to Cc ]
>
> On 25.07.2014 03:47, Linus Torvalds wrote:
>> On Wed, Jul 23, 2014 at 6:43 PM, Michel Dänzer <michel@...nzer.net> wrote:
>>>>
>>>> Michel, mind doing
>>>>
>>>>     make kernel/sched/fair.s
>>>>
>>>> and sending us the resulting file?
>>>
>>> Here it is, gzipped, hope that's okay.
>>>
>>> Note that my tree is now based on 3.16-rc6.
>>
>> Ok, so I'm looking at the code generation and your compiler is pure
>> and utter *shit*.
>>
>> Adding Jakub to the cc, because gcc-4.9.0 seems to be terminally broken.
>>
>> Lookie here, your compiler does some absolutely insane things with the
>> spilling, including spilling a *constant*. For chrissake, that
>> compiler shouldn't have been allowed to graduate from kindergarten.
>> We're talking "sloth that was dropped on the head as a baby" level
>> retardation levels here:
>>
>>         ...
>>         movq    $load_balance_mask, -136(%rbp)  #, %sfp
>>         subq    $184, %rsp      #,
>>         movq    (%rdx), %rax    # sd_22(D)->parent, sd_parent
>>         movl    %edi, -144(%rbp)        # this_cpu, %sfp
>>         movl    %ecx, -140(%rbp)        # idle, %sfp
>>         movq    %r8, -200(%rbp) # continue_balancing, %sfp
>>         movq    %rax, -184(%rbp)        # sd_parent, %sfp
>>         movq    -136(%rbp), %rax        # %sfp, tcp_ptr__
>> #APP
>>         add %gs:this_cpu_off, %rax      # this_cpu_off, tcp_ptr__
>> #NO_APP
>>         ...
>>
>> Note the contents of -136(%rbp). Seriously. That's an
>> _immediate_constant_ that the compiler is spilling.
>>
>> Somebody needs to raise that as a gcc bug. Because it damn well is
>> some seriously crazy shit.
>>
>> However, that constant spilling part just counts as "too stupid to
>> live". The real bug is this:
>>
>>         movq    $load_balance_mask, -136(%rbp)  #, %sfp
>>         subq    $184, %rsp      #,
>>
>> where gcc creates the stack frame *after* having already used it to
>> save that constant *deep* below the stack frame.
>>
>> The x86-64 ABI specifies a 128-byte red-zone under the stack pointer,
>> and this is ok by that limit. It looks like it's illegal (136 > 128),
>> but the fact is, we've had four "pushq"s to update %rsp since loading
>> the frame pointer, so it's just *barely* legal with the red-zoning.
>>
>> But we build the kernel with -mno-red-zone. We do *not* follow the
>> x86-64 ABI wrt redzoning, because we *cannot*: interrupts while in
>> kernel mode *will* use the stack without a redzone. So that
>> "-mno-red-zone" is not some "optional guideline". It's a hard and
>> harsh requirement for the kernel, and gcc-4.9 is a buggy piece of shit
>> for ignoring it. And your bug happens becuase you happen to hit an
>> interrupt _just_ in that single instruction window (or perhaps hit
>> some other similar case and corrupted kernel data structures earlier).
>>
>> Now, I suspect that this redzoning bug might actually be related to
>> the fact that gcc is stupid in spilling a constant. I would not be
>> surprised if there is some liveness analysis going on to decide *when*
>> to insert the stack decrement, and constants are being ignored because
>> clearly liveness isn't an issue for a constant value. So the two bugs
>> ("stupid constant spilling" and "invalid use or red zone stack") go
>> hand in hand. But who knows.
>>
>> Anyway, this is not a kernel bug. This is your compiler creating
>> completely broken code. We may need to add a warning to make sure
>> nobody compiles with gcc-4.9.0, and the Debian people should probably
>> downgrate their shiny new compiler.
>
> Attached is fair.s from Debian gcc 4.8.3-5. Does that look better? I'm
> going to try reproducing the problem with a kernel built by that now.
>
>
> --
> Earthling Michel Dänzer                           http://www.amd.com
> Libre software enthusiast                      Mesa and X developer

Hey guys,
I am new so please bear with me here but I can build test this on
Ubuntu 14.04 with gcc 4.8.1.
I am wondering through after only speed reading these messages , if
you can just give me
a one line summary of what I need to be looking for :).
Nick
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