linux-kernel - correct locking in softirq

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Message-ID: <885896af0902262354v47ddf79ch3d62edaeb1e940bd@mail.gmail.com>
Date:	Fri, 27 Feb 2009 08:54:41 +0100
From:	Giacomo <delleceste@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: correct locking in softirq

Good morning

Harald Welte's "The journey of a packet through the Linux 2.6.10
network stack" article says that packet travelling inside
linux kernel 2.6 (the receive / input part) runs in softirq context.

Hooking with netfilter's hooks in a kernel module, i need to read for
each packet received a list of rules.

Since in input and prerouting hooks the context is softirq (perhaps
also in forward?), I need some read lock
feature.

I currently use RCU lists and, while reading lists I use

READ

read_lock_bh()

together with list_for_each_rcu()

When changing, or flushing, rules, I use

WRITE

spin_lock() + list_add_tail_rcu() (adding)

or spin_lock() + list_for_each_entry() (for listing and then freeing
with list_del_rcu() and call_rcu() )

The question is:

- is the read part above correct? - do I really need _bh()? or should
I use simply read_lock() ?

Thanks in advance

Giacomo


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