Re: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder Cookie
From: Eric Dumazet
Date: Mon Nov 02 2009 - 08:16:57 EST
William Allen Simpson a écrit :
> Eric Dumazet wrote:
>> cookie_hash() runs in a non preemptable context. CPU cannot change
>> under us.
>> (or else, we would not use __get_cpu_var(ipv4_cookie_scratch); )
>> And of course, each cpu gets its own scratch area, thanks to
> Interesting. I'm not sure that running CPU intensive functions like
> SHA1 in
> a non-preemptable context is a good idea. I'd assumed it wasn't!
> Perhaps you could point at the documentation in the code that explains
I suggest you read Documentations/ files about softirq
Large part of network code is run by softirq handler, and a softirq handler
is not preemptable with another softirq (including itself).
> Perhaps a function header comment that mentions it?
So we are going to add a header to thousand of functions repeating this prereq ?
> All I know is (from testing) that the tcp_minisockets.c caller is sometimes
> called in a fashion that requires atomic allocation, and other times
> does not!
Maybe callers have different contexts (running from softirq handler or
from process context). Atomic ops are expensive and we try to avoid them
> See my "Subject: query: tcpdump versus atomic?" thread from Oct 14th.
You probably add a bug in your kernel, leaving a function with unpaired lock/unlock
There are books about linux internals that you could read if you want some extra
documentation. Dont ask me details, I never read them :)
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