Re: Random panic in load_balance() with 3.16-rc

From: Nick Krause
Date: Thu Jul 24 2014 - 22:37:01 EST

On Thu, Jul 24, 2014 at 9:25 PM, Michel DÃnzer <michel@xxxxxxxxxxx> 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 DÃnzer <michel@xxxxxxxxxxx> 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 DÃnzer
> 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 :).
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