From: Stefan Rompf <stefan@loplof.de>
To: Arjan van de Ven <arjan@infradead.org>
Subject: Re: [2.6 patch] i386: always use 4k/4k stacks
Date: Sun, 18 Dec 2005 13:04:37 +0100
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Am Sonntag 18 Dezember 2005 12:21 schrieb Arjan van de Ven:

> the kernel has a stack overflow detector, which checks at irq entry time
> if the stack is "rather high" (7kb into the stack on a 8kb stack, 3.5kb
> on a 4k stack). When this warning hits there's still runway left (like
> 12.5 percent), but lets say the end becomes in sight. If the stack usage
> would be really tight, this "early warning" detector would be hitting a
> lot of people, right?

Wrong. The probability that an interrupt happens just during the codepath with 
highest stack usage is very small. Anyway CONFIG_DEBUG_STACKOVERFLOW is not 
enabled in 2.6.14.4 i386 defconfig. Don't know about vendor kernel kernels 
though.

I thought more about filling the stack with some arbitrary value on thread 
startup and checking how much has been overwritten on a regular basis. Part 
of it is alreay there, hidden unter CONFIG_DEBUG_STACK_USAGE. The 
verification should just happen timer-controlled, not only on sysrq-whatever.

> (and the "safety net" is a bit of misnomer, since it's not really safe,
> just "statistically different" if the shit hits the fan)

If you can't even guarantee that 8k (or 6k) is enough, how can you vote for 4k 
then ;-) Just a little provocation, I don't plan getting too involved into 
this dicussion, hell, this is just about a ridiculously small amount of self 
contained #ifdef'd code ;-)

Stefan
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