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

Jeff Garzik
2008-08-26T19:55:14Z

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