linux-kernel - Re: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock

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Date:	Tue, 13 Jul 2010 18:53:48 +0200
From:	Peter Palfrader <peter@...frader.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	stable@...nel.org, stable-review@...nel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Glauber Costa <glommer@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization
	point for pvclock

On Tue, 13 Jul 2010, Avi Kivity wrote:

>> The same:
>>
>> [git|v2.6.32.16] weasel@...lma:/scratch/kernel/2.6.32.16$ nm arch/x86/kernel/pvclock.o
>> 0000000000000000 r last_value
>>                   U native_read_tsc
>> 0000000000000040 T pvclock_clocksource_read
>> 0000000000000100 T pvclock_read_wallclock
>> 0000000000000000 T pvclock_tsc_khz
>>                   U set_normalized_timespec
>>    
>
> But it's
>
>>
>> static atomic64_t last_value = ATOMIC64_INIT(0);
>>
>
> Doesn't make any sense.
>
> Let's try to see if the toolchain is confused:
>
> - rename last_value to some random name

0000000000000000 r some_random_name

> - drop the 'static' qualifier

0000000000000000 B last_value

> - drop the '= ATOMIC64_INIT(0)'

0000000000000000 b last_value

> - all of the above

0000000000000000 B some_random_name




make -f scripts/Makefile.build obj=arch/x86/kernel arch/x86/kernel/pvclock.o
  gcc -Wp,-MD,arch/x86/kernel/.pvclock.o.d  -nostdinc -isystem /usr/lib/gcc/x86_64-linux-gnu/4.3.2/include -Iinclude  -I/scratch/kernel/2.6.32.16/arch/x86/include -include include/linux/autoconf.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m64 -mtune=generic -mno-red-zone -mcmodel=kernel -funit-at-a-time -maccumulate-outgoing-args -fstack-protector -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -fno-omit-frame-pointer -fno-optimize-sibling-calls -g -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow   -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(pvclock)"  -D"KBUILD_MODNAME=KBUILD_STR(pvclock)"  -c -o arch/x86/kernel/pvclock.o arch/x86/kernel/pvclock.c

[git|v2.6.32.16] weasel@...lma:/scratch/kernel/2.6.32.16$ gcc --version
gcc (Debian 4.3.2-1.1) 4.3.2
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Using -O0 also results in a writeable last_value (b).

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