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?
Perhaps a function header comment that mentions it?

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!

See my "Subject: query: tcpdump versus atomic?" thread from Oct 14th.