linux-kernel - Re: seqcount usage in xt_replace

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Date:   Thu, 10 Jan 2019 13:38:11 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Anatol Pomozov <anatol.pomozov@...il.com>,
        Florian Westphal <fw@...len.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: seqcount usage in xt_replace_table()

On Thu, Jan 10, 2019 at 1:30 PM Andrea Parri
<andrea.parri@...rulasolutions.com> wrote:
>
> > For seqcounts we currently simply ignore all accesses within the read
> > section (thus the requirement to dynamically track read sections).
> > What does LKMM say about seqlocks?
>
> LKMM does not currently model seqlocks, if that's what you're asking;
> c.f., tools/memory-model/linux-kernel.def for a list of the currently
> supported synchronization primitives.
>
> LKMM has also no notion of "data race", it insists that the code must
> contain no unmarked accesses; we have been discussing such extensions
> since at least Dec'17 (we're not quite there!, as mentioned by Paul).

How does it call cases that do contain unmarked accesses then? :)

> My opinion is that ignoring all accesses within a given read section
> _can_ lead to false negatives

Absolutely. But this is a deliberate decision.
For our tools we consider priority 1: no false positives. Period.
Priority 2: also report some true positives in best effort manner.

> (in every possible definition of "data
> race" and "read sections" I can think of at the moment ;D):
>
>         P0                              P1
>         read_seqbegin()                 x = 1;
>         r0 = x;
>         read_seqretry() // =0
>
> ought to be "racy"..., right?  (I didn't audit all the callsites for
> read_{seqbegin,seqretry}(), but I wouldn't be surprised to find such
> pattern ;D ... "legacy", as you recalled).
>
>   Andrea