linux-kernel - Re: [PATCH] sched, time: fix build error with 64 bit cputime

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Date:	Tue, 30 Sep 2014 15:19:19 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	umgwanakikbuti@...il.com, fweisbec@...il.com,
	akpm@...ux-foundation.org, srao@...hat.com, lwoodman@...hat.com,
	atheurer@...hat.com, oleg@...hat.com,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] sched, time: fix build error with 64 bit cputime_t on
 32 bit systems

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On 09/30/2014 02:49 PM, Arnd Bergmann wrote:
> On Tuesday 30 September 2014 13:40:11 Rik van Riel wrote:
>> On Tue, 30 Sep 2014 13:56:37 +0200 Arnd Bergmann <arnd@...db.de>
>> wrote:
>> 
>>> A recent change to update the stime/utime members of
>>> task_struct using atomic cmpxchg broke configurations on 32-bit
>>> machines with CONFIG_VIRT_CPU_ACCOUNTING_GEN set, because that
>>> uses 64-bit nanoseconds, leading to a link-time error:
>>> 
>>> kernel/built-in.o: In function `cputime_adjust': 
>>> :(.text+0x25234): undefined reference to `__bad_cmpxchg'
>> 
>> Arnd, this should fix your problem, while still ensuring that the
>> cpu time counters only ever go forward.
>> 
>> I do not have cross compiling toolchains set up here, but I
>> assume this fixes your bug.
>> 
>> Ingo & Peter, if this patch fixes the bug for Arnd, could you
>> please merge it into -tip?
>> 
>> Linus, the changeset causing the problem is only in -tip right
>> now, and this patch will not apply to your tree.
> 
> It compiles and links on arm32 with both 32-bit and 64-bit
> cputime_t, but I now get a new warning for the former case:
> 
> kernel/sched/cputime.c: In function 'cputime_advance': 
> kernel/sched/cputime.c:576:29: warning: passing argument 1 of
> '__cmpxchg64_mb' from incompatible pointer type cmpxchg64(counter,
> old, new); ^ In file included from
> /git/arm-soc/arch/arm/include/asm/atomic.h:19:0, from
> /git/arm-soc/include/linux/atomic.h:4, from
> /git/arm-soc/include/linux/debug_locks.h:5, from
> /git/arm-soc/include/linux/lockdep.h:23, from
> /git/arm-soc/include/linux/spinlock_types.h:18, from
> /git/arm-soc/include/linux/spinlock.h:81, from
> /git/arm-soc/include/linux/seqlock.h:35, from
> /git/arm-soc/include/linux/time.h:5, from
> /git/arm-soc/include/uapi/linux/timex.h:56, from
> /git/arm-soc/include/linux/timex.h:56, from
> /git/arm-soc/include/linux/sched.h:19, from
> /git/arm-soc/kernel/sched/cputime.c:2: 
> /git/arm-soc/arch/arm/include/asm/cmpxchg.h:255:105: note: expected
> 'long long unsigned int *' but argument is of type 'cputime_t *' 
> static inline unsigned long long __cmpxchg64_mb(unsigned long long
> *ptr, kernel/sched/cputime.c:576:5: warning: value computed is not
> used [-Wunused-value] cmpxchg64(counter, old, new); ^

Is u64 on arm defined to "long long unsigned int", and is the compiler
really blowing up over the difference between that and "unsigned long
long"?

Is u64 on arm really supposed to be "long long unsigned int"?

Am I overlooking something?

> I suspect there is no solution that doesn't involve the
> preprocessor. How about adding a cputime_cmpxchg() helper next to
> the cputime_t definition?

I thought of that first, and quickly dismissed it with "yuck" :)

I guess we could cast cputime_t to unsigned long long in the branch
that calls cmpxchg64.  Is that acceptable/unacceptable to people?

- -- 
All rights reversed
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