lkml.org 
[lkml]   [2009]   [Nov]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder Cookie
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
>> __get_cpu_var()
>>
> 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
> this?

I suggest you read Documentations/ files about softirq

http://docs.blackfin.uclinux.org/kernel/generated/kernel-hacking.xml


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
if/when possible.

>
> 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
of notallow_something/allow_something

There are books about linux internals that you could read if you want some extra
documentation. Dont ask me details, I never read them :)

--
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/

\
 
 \ /
  Last update: 2009-11-02 14:19    [W:1.814 / U:0.012 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site