linux-kernel - Re: [PATCH v2 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure

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Date:   Wed, 23 Oct 2019 14:32:20 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Alexander Potapenko <glider@...gle.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Borislav Petkov <bp@...en8.de>, Daniel Axtens <dja@...ens.net>,
        Daniel Lustig <dlustig@...dia.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Howells <dhowells@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Joel Fernandes <joel@...lfernandes.org>,
        Jonathan Corbet <corbet@....net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Mark Rutland <mark.rutland@....com>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-efi@...r.kernel.org,
        "open list:KERNEL BUILD + fi..." <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v2 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure

On Thu, Oct 17, 2019 at 4:13 PM Marco Elver <elver@...gle.com> wrote:
>
> Kernel Concurrency Sanitizer (KCSAN) is a dynamic data-race detector for
> kernel space. KCSAN is a sampling watchpoint-based data-race detector.
> See the included Documentation/dev-tools/kcsan.rst for more details.

I think there is some significant potential for improving performance.
Currently we have __tsan_read8 do 2 function calls, push/pop, the
second call is on unpredicted slow path.
Then __kcsan_check_watchpoint and __kcsan_setup_watchpoint do full
load of spills and lots of loads and checks that are not strictly
necessary or can be avoided. Additionally __kcsan_setup_watchpoint
calls non-inlined kcsan_is_atomic.
I think we need to try to structure it around the fast path as follows:
__tsan_read8 does no function calls and no spills on fast path for
both checking existing watchpoints and checking if a new watchpoint
need to be setup. If it discovers a race with existing watchpoint or
needs to setup a new one, that should be non-inlined tail calls to the
corresponding slow paths.
In particular, global enable/disable can be replaced with
occupying/freeing all watchpoints.
Per cpu disabled check should be removed from fast path somehow, it's
only used around debugging checks or during reporting. There should be
a way to check it on a slower path.
user_access_save should be removed from fast path, we needed it only
if we setup a watchpoint. But I am not sure why we need it at all, we
should not be reading any user addresses.
should_watch should be restructured to decrement kcsan_skip first, if
it hits zero (with unlikely hint), we go to slow path. The slow path
resets kcsan_skip to something random. The comment mentions
prandom_u32 is too expensive, do I understand it correctly that you
tried to call it on the fast path? I would expect it is fine for slow
path and will give us better randomness.
At this point we should return from __tsan_read8.

To measure performance we could either do some synthetic in-kernel
benchmarks (e.g. writing something to the debugfs file, which will do
a number of memory accesses in a loop). Or you may try these
user-space benchmarks:
https://github.com/google/sanitizers/blob/master/address-sanitizer/kernel_buildbot/slave/bench_readv.c
https://github.com/google/sanitizers/blob/master/address-sanitizer/kernel_buildbot/slave/bench_pipes.c