linux-kernel - Re: seqcount usage in xt_replace

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Date:   Thu, 10 Jan 2019 06:50:29 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.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 01:38:11PM +0100, Dmitry Vyukov wrote:
> 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).

One approach would be to forgive data races in the seqlock read-side
critical section only if:

o	There was a later matching read_seqretry() that returned true, and

o	There were no dereferences of any data-racy load.  (Yeah, this
	one should be good clean fun to model!)

Do people nest read_seqbegin(), and if so, what does that mean?  ;-)

							Thanx, Paul