linux-kernel - Re: seqcount usage in xt_replace

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Message-ID: <20190110124427.GB21224@hirez.programming.kicks-ass.net>
Date:   Thu, 10 Jan 2019 13:44:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Anatol Pomozov <anatol.pomozov@...il.com>
Cc:     fw@...len.de, Dmitry Vyukov <dvyukov@...gle.com>,
        paulmck@...ux.ibm.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()

On Tue, Jan 08, 2019 at 11:33:39AM -0800, Anatol Pomozov wrote:
> Hello folks,
> 
> A bit of context what I am doing. I am trying to port KTSAN (Kernel
> Thread Sanitizer) tool to v4.20. That tool tracks shared data usage
> and makes sure it is accessed in a thread-safe manner.
> 
> seqlock is a synchronization primitive used by Linux kernel. KTSAN
> annotates read_seqbegin()/read_seqretry() and tracks what data been
> accessed in its critical section.
> 
> During KTSAN port I found and interesting seqcount usage introduced in
> commit 80055dab5de0c8677bc148c4717ddfc753a9148e
> 
> If I read this commit correctly xt_replace_table() does not use
> seqlock in a canonical way to specify a critical section. Instead the
> code reads the counter and waits until it gets to a specific value.

(gets away from)

> Now I want KTSAN to play with this code nicely. I need to tell KTSAN
> something like "this raw_read_seqcount() does not start a critical
> section, just ignore it". So temporary I introduced
> raw_read_seqcount_nocritical() function that is ignored by KTSAN. Is
> it a good solution?

This code is special enough to just do: READ_ONCE(->sequence) and be
done with it. It doesn't need the smp_rmb() or anything else.