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Message-ID: <20200513111057.GN2957@hirez.programming.kicks-ass.net>
Date: Wed, 13 May 2020 13:10:57 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Marco Elver <elver@...gle.com>
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>, dvyukov@...gle.com
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen
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) \
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