Messages in this thread | | | Date | Thu, 24 Jul 2014 11:47:17 -0700 | Subject | Re: Random panic in load_balance() with 3.16-rc | From | Linus Torvalds <> |
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On Wed, Jul 23, 2014 at 6:43 PM, Michel Dänzer <michel@daenzer.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.
Jakub, any ideas?
Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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