linux-kernel - Re: seqcount usage in xt_replace

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Date:   Thu, 10 Jan 2019 13:54:41 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Anatol Pomozov <anatol.pomozov@...il.com>,
        Florian Westphal <fw@...len.de>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()

On Thu, Jan 10, 2019 at 1:44 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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.

Sounds good to me.
>From KTSAN perspective it then should work without any additional
dance, it's always good when code works as-is.