linux-kernel - Re: seqcount usage in xt_replace

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Date:   Thu, 10 Jan 2019 14:25:20 +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 1:47 PM Andrea Parri
<andrea.parri@...rulasolutions.com> wrote:
>
> 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? :)
>
> "work-in-progress" ;), or "limitation" (see tools/memory-model/README)

Let's call it /data race/ interim then :)
Which also have literally undefined behavior in LKMM. Which now
precisely matches the implementation language (C) definitions. Which
is nice.

> > > 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.
>
> This sound reasonable to me.  But please don't overlook the fact that
> to be able to talk about "false positive" and "false negative" (for a
> data race detector) we need to agree about "what a data race is".

Having a formal model would be undoubtedly good.
But in practice things are much simpler. The complex cases that
majority of LKMM deals with are <<1% of kernel concurrency. The
majority of kernel cases are "no concurrent accesses at all", "always
protected by a mutex", "passed as argument to a new thread", "the
canonical store-release/load-acquire synchronization". For these I
hope there is no controversy across C, POSIX, gcc, clang, kernel.
Handling these cases in a race detector brings 99.9% of benefit. And
for more complex cases (like seqlock) we can always approximate as "no
races there" which inevitably satisfy our priorities (if you report
nothing, you don't report false positives).

But I am much more concerned about actual kernel code and behavior wrt
a memory model. We are talking about interaction between LKMM <->
KTSAN. When a way more important question is LKMM <-> actual kernel
behavior. KTSAN is really a secondary thing in this picture. So if
anything needs a memory model, or needs to be blocked on a memory
model, that's writing kernel code ;)

> (The hope, of course, is that the LKMM will have a say soon here ...)
>
>   Andrea
>
>
> >
> > > (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