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Date:   Wed, 13 May 2020 13:48:41 +0200
From:   Marco Elver <elver@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen

On Wed, 13 May 2020 at 13:11, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, May 12, 2020 at 10:31:44PM +0200, Marco Elver wrote:
> > On Tue, 12 May 2020 at 21:08, Peter Zijlstra <peterz@...radead.org> wrote:
>
> > > data_race() will include active calls to kcsan_{dis,en}able_current(),
> > > and this must not happen.
> >
> > Only if instrumentation is enabled for the compilation unit. If you
> > have KCSAN_SANITIZE_foo.c := n, no calls are emitted not even to
> > kcsan_{dis,en}able_current(). Does that help?
> >
> > By default, right now __READ_ONCE() will still generate a call due to
> > instrumentation (call to __tsan_readX).
>
> Ah, so looking at:
>
> #define data_race(expr)                                                 \
> ({                                                                      \
>         __kcsan_disable_current();                                      \
>         ({                                                              \
>                 __unqual_scalar_typeof(({ expr; })) __v = ({ expr; });  \
>                 __kcsan_enable_current();                               \
>                 __v;                                                    \
>         });                                                             \
> })
>
> had me confused, but then you've got this squirreled away in another
> header:
>
> #ifdef __SANITIZE_THREAD__
> /*
>  * Only calls into the runtime when the particular compilation unit has KCSAN
>  * instrumentation enabled. May be used in header files.
>  */
> #define kcsan_check_access __kcsan_check_access
>
> /*
>  * Only use these to disable KCSAN for accesses in the current compilation unit;
>  * calls into libraries may still perform KCSAN checks.
>  */
> #define __kcsan_disable_current kcsan_disable_current
> #define __kcsan_enable_current kcsan_enable_current_nowarn
> #else
> static inline void kcsan_check_access(const volatile void *ptr, size_t size,
>                                       int type) { }
> static inline void __kcsan_enable_current(void)  { }
> static inline void __kcsan_disable_current(void) { }
> #endif
>
> And I suppose KCSAN_SANITIZE := n, results in __SANITIZE_THREAD__ not
> being defined.
>
> I really hate the function attribute situation, that is some ill
> considered trainwreck.
>
> Looking at this more, I found you already have:
>
> arch/x86/kernel/Makefile:KCSAN_SANITIZE := n
> arch/x86/kernel/Makefile:KCOV_INSTRUMENT                := n
> arch/x86/mm/Makefile:KCSAN_SANITIZE := n
>
> So how about I complete that and kill everhthing for all arch/x86/ that
> has DEFINE_IDTENTRY*() in.
>
> That avoids me having to do a lot of work to split up the tricky bits.
> You didn't think it was important, so why should I bother.
>
> So then I end up with something like the below, and I've validated that
> does not generate instrumentation... HOWEVER, I now need ~10g of memory
> and many seconds to compile each file in arch/x86/kernel/.
>
> That is, when I do 'make arch/x86/kernel/ -j8', it is slow enough that I
> can run top and grab:
>
> 31249 root      20   0 6128580   4.1g  13092 R 100.0  13.1   0:16.29 cc1
> 31278 root      20   0 6259456   4.4g  12932 R 100.0  13.9   0:16.27 cc1
> 31286 root      20   0 7243160   4.9g  13028 R 100.0  15.5   0:16.26 cc1
> 31289 root      20   0 5933824   4.0g  12936 R 100.0  12.8   0:16.26 cc1
> 31331 root      20   0 4250924   2.9g  13016 R 100.0   9.3   0:09.54 cc1
> 31346 root      20   0 1939552   1.3g  13028 R 100.0   4.1   0:07.01 cc1
> 31238 root      20   0 6293524   4.1g  13008 R 100.0  13.0   0:16.29 cc1
> 31259 root      20   0 6817076   4.7g  12956 R 100.0  14.9   0:16.27 cc1
>
> and it then triggers OOMs, while previously I could build kernels with
> -j80 on that machine:
>
> 31289 root      20   0   10.8g   6.2g    884 R 100.0  19.7   1:01.56 cc1
> 31249 root      20   0   10.2g   6.1g    484 R 100.0  19.3   1:00.10 cc1
> 31331 root      20   0   10.3g   7.2g    496 R 100.0  23.1   0:53.95 cc1
>
> Only 3 left, because the others OOM'ed.
>
> This is gcc-8.3, the situation with gcc-10 seems marginally better, but
> still atrocious.
>
> ---
> diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
> index b7a5790d8d63..ff959f0209e7 100644
> --- a/arch/x86/entry/Makefile
> +++ b/arch/x86/entry/Makefile
> @@ -6,6 +6,7 @@
>  KASAN_SANITIZE := n
>  UBSAN_SANITIZE := n
>  KCOV_INSTRUMENT := n
> +KCSAN_INSTRUMENT := n
>
>  CFLAGS_REMOVE_common.o = $(CC_FLAGS_FTRACE) -fstack-protector -fstack-protector-strong
>  CFLAGS_REMOVE_syscall_32.o = $(CC_FLAGS_FTRACE) -fstack-protector -fstack-protector-strong
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index d6d61c4455fa..f2a46a87026e 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -22,15 +22,18 @@ CFLAGS_REMOVE_early_printk.o = -pg
>  CFLAGS_REMOVE_head64.o = -pg
>  endif
>
> -KASAN_SANITIZE_head$(BITS).o                           := n
> -KASAN_SANITIZE_dumpstack.o                             := n
> -KASAN_SANITIZE_dumpstack_$(BITS).o                     := n
> -KASAN_SANITIZE_stacktrace.o                            := n
> -KASAN_SANITIZE_paravirt.o                              := n
> -
> -# With some compiler versions the generated code results in boot hangs, caused
> -# by several compilation units. To be safe, disable all instrumentation.
> -KCSAN_SANITIZE := n
> +#
> +# You cannot instrument entry code, that results in definite problems.
> +# In particular, anything with DEFINE_IDTENTRY*() in must not have
> +# instrumentation on.
> +#
> +# If only function attributes and inlining would work properly, without
> +# that untangling this is a giant trainwreck, don't attempt.
> +#
> +KASAN_SANITIZE := n
> +UBSAN_SANITIZE := n
> +KCOV_INSTRUMENT := n
> +KCSAN_INSTRUMENT := n
>
>  OBJECT_FILES_NON_STANDARD_test_nx.o                    := y
>  OBJECT_FILES_NON_STANDARD_paravirt_patch.o             := y
> @@ -39,11 +42,6 @@ ifdef CONFIG_FRAME_POINTER
>  OBJECT_FILES_NON_STANDARD_ftrace_$(BITS).o             := y
>  endif
>
> -# If instrumentation of this dir is enabled, boot hangs during first second.
> -# Probably could be more selective here, but note that files related to irqs,
> -# boot, dumpstack/stacktrace, etc are either non-interesting or can lead to
> -# non-deterministic coverage.
> -KCOV_INSTRUMENT                := n
>
>  CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace
>
> diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
> index f7fd0e868c9c..f8d7e7432847 100644
> --- a/arch/x86/mm/Makefile
> +++ b/arch/x86/mm/Makefile
> @@ -1,15 +1,17 @@
>  # SPDX-License-Identifier: GPL-2.0
> -# Kernel does not boot with instrumentation of tlb.c and mem_encrypt*.c
> -KCOV_INSTRUMENT_tlb.o                  := n
> -KCOV_INSTRUMENT_mem_encrypt.o          := n
> -KCOV_INSTRUMENT_mem_encrypt_identity.o := n
>
> -KASAN_SANITIZE_mem_encrypt.o           := n
> -KASAN_SANITIZE_mem_encrypt_identity.o  := n
> -
> -# Disable KCSAN entirely, because otherwise we get warnings that some functions
> -# reference __initdata sections.
> -KCSAN_SANITIZE := n
> +#
> +# You cannot instrument entry code, that results in definite problems.
> +# In particular, anything with DEFINE_IDTENTRY*() in must not have
> +# instrumentation on.
> +#
> +# If only function attributes and inlining would work properly, without
> +# that untangling this is a giant trainwreck, don't attempt.
> +#
> +KASAN_SANITIZE := n
> +UBSAN_SANITIZE := n
> +KCOV_INSTRUMENT := n
> +KCSAN_INSTRUMENT := n
>
>  ifdef CONFIG_FUNCTION_TRACER
>  CFLAGS_REMOVE_mem_encrypt.o            = -pg
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 3bb962959d8b..48f85d1d2db6 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -241,7 +241,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>   * atomicity or dependency ordering guarantees. Note that this may result
>   * in tears!
>   */
> -#define __READ_ONCE(x) (*(const volatile __unqual_scalar_typeof(x) *)&(x))
> +#define __READ_ONCE(x) data_race((*(const volatile __unqual_scalar_typeof(x) *)&(x)))
>
>  #define __READ_ONCE_SCALAR(x)                                          \
>  ({                                                                     \
> @@ -260,7 +260,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>
>  #define __WRITE_ONCE(x, val)                                           \
>  do {                                                                   \
> -       *(volatile typeof(x) *)&(x) = (val);                            \
> +       data_race(*(volatile typeof(x) *)&(x) = (val));                 \
>  } while (0)
>
>  #define __WRITE_ONCE_SCALAR(x, val)                                    \
>

Disabling most instrumentation for arch/x86 is reasonable. Also fine
with the __READ_ONCE/__WRITE_ONCE changes (your improved
compiler-friendlier version).

We likely can't have both: still instrument __READ_ONCE/__WRITE_ONCE
(as Will suggested) *and* avoid double-instrumentation in arch_atomic.
If most use-cases of __READ_ONCE/__WRITE_ONCE are likely to use
data_race() or KCSAN_SANITIZE := n anyway, I'd say it's reasonable for
now.

Thanks,
-- Marco

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