linux-kernel - Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar

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Date:   Thu, 28 May 2020 17:16:09 +0200
From:   Marco Elver <elver@...gle.com>
To:     sedat.dilek@...il.com
Cc:     Arnd Bergmann <arnd@...db.de>, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar_typeof
 compilation time

On Thu, 28 May 2020 at 04:12, Sedat Dilek <sedat.dilek@...il.com> wrote:
>
[...]

> > > >
> > > > In general, CONFIG_KCSAN=y and the defaults for the other KCSAN
> > > > options should be good. Depending on the size of your system, you
> > > > could also tweak KCSAN runtime performance:
> > > > https://lwn.net/Articles/816850/#Interacting%20with%20KCSAN%20at%20Runtime
> > > > -- the defaults should be good for most systems though.
> > > > Hope this helps. Any more questions, do let me know.
> > > >
> > >
> > > Which "projects" and packages do I need?
> > >
> > > I have installed:
> > >
> > > # LC_ALL=C apt-get install llvm-11 clang-11 lld-11
> > > --no-install-recommends -t llvm-toolchain -y
> > >
> > > # dpkg -l | grep
> > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261 | awk
> > > '/^ii/ {print $1 " " $2 " " $3}' | column -t
> > > ii  clang-11
> > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > ii  libclang-common-11-dev
> > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > ii  libclang-cpp11
> > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > ii  libclang1-11
> > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > ii  libllvm11:amd64
> > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > ii  lld-11
> > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > ii  llvm-11
> > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > ii  llvm-11-runtime
> > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > >
> > > Is that enough?
> >
> > Just clang-11 (and its transitive dependencies) is enough. Unsure what
> > your installed binary is, likely "clang-11", so if you can do "make
> > CC=clang-11 defconfig" (and check for CONFIG_HAVE_KCSAN_COMPILER)
> > you're good to go.
> >
>
> I was able to build with clang-11 from apt.llvm.org.
>
> [ build-time ]
>
> Normally, it takes me approx. 05:00 to build with clang-10
> (10.0.1-rc1) and Linux v5.7-rc7.
>
> This time start: 21:18 and stop: 03:45 means 06:27 - took 01:27 longer.
>
> Samsung Ultrabook 2nd generation aka Intel Sandybridge CPU with 'make -j3'.
>
> [ diffconfig ]
>
>  BUILD_SALT "5.7.0-rc7-2-amd64-clang" -> "5.7.0-rc7-3-amd64-clang"
>  CLANG_VERSION 100001 -> 110000
> +CC_HAS_ASM_INLINE y
> +HAVE_ARCH_KCSAN y
> +HAVE_KCSAN_COMPILER y
> +KCSAN y
> +KCSAN_ASSUME_PLAIN_WRITES_ATOMIC y
> +KCSAN_DEBUG n
> +KCSAN_DELAY_RANDOMIZE y
> +KCSAN_EARLY_ENABLE y
> +KCSAN_IGNORE_ATOMICS n
> +KCSAN_INTERRUPT_WATCHER n
> +KCSAN_NUM_WATCHPOINTS 64
> +KCSAN_REPORT_ONCE_IN_MS 3000
> +KCSAN_REPORT_RACE_UNKNOWN_ORIGIN y
> +KCSAN_REPORT_VALUE_CHANGE_ONLY y
> +KCSAN_SELFTEST y
> +KCSAN_SKIP_WATCH 4000
> +KCSAN_SKIP_WATCH_RANDOMIZE y
> +KCSAN_UDELAY_INTERRUPT 20
> +KCSAN_UDELAY_TASK 80
>
> I am seeing this data-races:
>
> root@...za:~# LC_ALL=C dmesg -T | grep 'BUG: KCSAN: data-race'
> [Thu May 28 03:51:53 2020] BUG: KCSAN: data-race in
> mutex_spin_on_owner+0xe0/0x1b0
> [Thu May 28 03:52:00 2020] BUG: KCSAN: data-race in mark_page_accessed
> / workingset_activation
> [Thu May 28 03:52:02 2020] BUG: KCSAN: data-race in
> mutex_spin_on_owner+0xe0/0x1b0
> [Thu May 28 03:52:08 2020] BUG: KCSAN: data-race in
> blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> [Thu May 28 03:52:10 2020] BUG: KCSAN: data-race in dd_has_work /
> dd_insert_requests
> [Thu May 28 03:52:11 2020] BUG: KCSAN: data-race in
> mutex_spin_on_owner+0xe0/0x1b0
> [Thu May 28 03:52:13 2020] BUG: KCSAN: data-race in
> page_counter_try_charge / page_counter_try_charge
> [Thu May 28 03:52:15 2020] BUG: KCSAN: data-race in ep_poll_callback /
> ep_send_events_proc
> [Thu May 28 03:52:21 2020] BUG: KCSAN: data-race in
> mutex_spin_on_owner+0xe0/0x1b0
> [Thu May 28 03:52:25 2020] BUG: KCSAN: data-race in
> mutex_spin_on_owner+0xe0/0x1b0
> [Thu May 28 03:52:26 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 03:52:31 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 03:52:38 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 03:52:53 2020] BUG: KCSAN: data-race in dd_has_work /
> dd_insert_requests
> [Thu May 28 03:52:56 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 03:52:59 2020] BUG: KCSAN: data-race in
> blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> [Thu May 28 03:53:25 2020] BUG: KCSAN: data-race in
> rwsem_spin_on_owner+0x102/0x1a0
> [Thu May 28 03:53:25 2020] BUG: KCSAN: data-race in
> page_counter_try_charge / page_counter_try_charge
> [Thu May 28 03:53:39 2020] BUG: KCSAN: data-race in do_epoll_wait /
> ep_poll_callback
> [Thu May 28 03:53:39 2020] BUG: KCSAN: data-race in find_next_and_bit+0x30/0xd0
> [Thu May 28 03:53:41 2020] BUG: KCSAN: data-race in dd_has_work /
> dd_insert_requests
> [Thu May 28 03:53:43 2020] BUG: KCSAN: data-race in do_epoll_wait /
> ep_poll_callback
> [Thu May 28 03:53:45 2020] BUG: KCSAN: data-race in dd_has_work /
> dd_insert_requests
> [Thu May 28 03:53:46 2020] BUG: KCSAN: data-race in
> blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> [Thu May 28 03:53:47 2020] BUG: KCSAN: data-race in
> rwsem_spin_on_owner+0x102/0x1a0
> [Thu May 28 03:54:02 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 03:54:11 2020] BUG: KCSAN: data-race in find_next_and_bit+0x30/0xd0
> [Thu May 28 03:54:19 2020] BUG: KCSAN: data-race in
> rwsem_spin_on_owner+0x102/0x1a0
> [Thu May 28 03:55:00 2020] BUG: KCSAN: data-race in
> mutex_spin_on_owner+0xe0/0x1b0
> [Thu May 28 03:56:14 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 03:56:50 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 03:56:50 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 03:56:52 2020] BUG: KCSAN: data-race in
> tick_nohz_next_event / tick_nohz_stop_tick
> [Thu May 28 03:56:58 2020] BUG: KCSAN: data-race in
> blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> [Thu May 28 03:57:58 2020] BUG: KCSAN: data-race in
> blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> [Thu May 28 03:58:00 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 03:58:07 2020] BUG: KCSAN: data-race in
> tick_nohz_next_event / tick_nohz_stop_tick
> [Thu May 28 03:58:44 2020] BUG: KCSAN: data-race in
> mutex_spin_on_owner+0xe0/0x1b0
> [Thu May 28 03:58:49 2020] BUG: KCSAN: data-race in __bitmap_subset+0x38/0xd0
> [Thu May 28 03:59:46 2020] BUG: KCSAN: data-race in
> tick_nohz_next_event / tick_nohz_stop_tick
> [Thu May 28 04:00:25 2020] BUG: KCSAN: data-race in dd_has_work /
> deadline_remove_request
> [Thu May 28 04:00:26 2020] BUG: KCSAN: data-race in
> tick_nohz_next_event / tick_nohz_stop_tick
>
> Full dmesg output and linux-config attached.

Thank you for the report. There are a number of known data races. Note
that, we do not think it's wise to rush fixes for data races,
especially because each one requires careful analysis of what the
appropriate response is. In the meantime, also have a look at these 2
articles (if you haven't already), which describes the current state
of things:

1. https://lwn.net/Articles/816850/
2. https://lwn.net/Articles/816854/

Thanks,
-- Marco