linux-kernel - Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c

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Date:	Tue, 26 Aug 2008 15:55:14 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	"Alan D. Brunelle" <Alan.Brunelle@...com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c - bisected

Linus Torvalds wrote:
> The downsides of inlining are big enough from both a debugging and a real 
> code generation angle (eg stack usage like this), that the upsides 
> (_somesimes_ smaller kernel, possibly slightly faster code) simply aren't 
> relevant.
> 
> So the "noinline" was random, yes, but this is a real issue. Looking at 
> checkstack output for a saner config (NR_CPUS=16), the top entries for me 
> are things like
> 
> 	ide_generic_init [vmlinux]:             1384
> 	idefloppy_ioctl [vmlinux]:              1208
> 	e1000_check_options [vmlinux]:  	1152
> 	...
> 
> which are "leaf" functions. They are broken as hell (the e1000 is 
> apparently because it builds structs on the stack that should all be 
> "static const", for example), but they are different from something like 
> the module init sequence in that they are not going to affect anything 
> else.


e1000_check_options builds a struct (singular) on the stack, really... 
struct e1000_option is reasonably small.

The problem, which has also shown itself in large ioctl-style switch{} 
statements, is that gcc will generate code such that the stack usage 
from independent code branches

	if {cond1} {
		char buster1[1000];
		foo(buster1);
	} else if (cond2) {
		char buster2[1000];
		foo(buster2);
	}

are added together, not noticed as mutually exclusive.

Of course, adding 'static const' as you noted is a reasonable 
workaround, but gcc is really annoying WRT stack allocation in this manner.

I had problems in the past, before struct ethtool_ops, with like ethtool 
ioctl switch statements using gobs of stack.  In fact, that was a big 
motivation for struct ethtool_ops.

	Jeff


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