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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Sat, 5 Oct 2019 06:16:48 +0200
Message-ID: <CACT4Y+bdPKQDGag1rZG6mCj2EKwEsgWdMuHZq_um2KuWOrog6Q@mail.gmail.com>
Subject: Re: Kernel Concurrency Sanitizer (KCSAN)
To:     Eric Dumazet <eric.dumazet@gmail.com>
Cc:     Will Deacon <will@kernel.org>, Marco Elver <elver@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Alexander Potapenko <glider@google.com>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Paul Turner <pjt@google.com>, Daniel Axtens <dja@axtens.net>,
        Anatol Pomazau <anatol@google.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>
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On Sat, Oct 5, 2019 at 2:58 AM Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > This one is tricky. What I think we need to avoid is an onslaught of
> > patches adding READ_ONCE/WRITE_ONCE without a concrete analysis of the
> > code being modified. My worry is that Joe Developer is eager to get their
> > first patch into the kernel, so runs this tool and starts spamming
> > maintainers with these things to the point that they start ignoring KCSAN
> > reports altogether because of the time they take up.
> >
> > I suppose one thing we could do is to require each new READ_ONCE/WRITE_ONCE
> > to have a comment describing the racy access, a bit like we do for memory
> > barriers. Another possibility would be to use atomic_t more widely if
> > there is genuine concurrency involved.
> >
>
> About READ_ONCE() and WRITE_ONCE(), we will probably need
>
> ADD_ONCE(var, value)  for arches that can implement the RMW in a single instruction.
>
> WRITE_ONCE(var, var + value) does not look pretty, and increases register pressure.

FWIW modern compilers can handle this if we tell them what we are trying to do:

void foo(int *p, int x)
{
    x += __atomic_load_n(p, __ATOMIC_RELAXED);
    __atomic_store_n(p, x, __ATOMIC_RELAXED);
}

$ clang test.c -c -O2 && objdump -d test.o

0000000000000000 <foo>:
   0: 01 37                add    %esi,(%rdi)
   2: c3                    retq

We can have syntactic sugar on top of this of course.