linux-kernel - Re: [patch 2.6.19-rc6] Stop gcc 4.1.0 optimizing wait_hpet

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Date:	Wed, 29 Nov 2006 14:56:20 +1100
From:	Keith Owens <kaos@....com.au>
To:	Nicholas Miell <nmiell@...cast.net>
cc:	linux-kernel@...r.kernel.org
Subject: Re: [patch 2.6.19-rc6] Stop gcc 4.1.0 optimizing wait_hpet_tick away 

Nicholas Miell (on Tue, 28 Nov 2006 19:08:25 -0800) wrote:
>On Wed, 2006-11-29 at 13:22 +1100, Keith Owens wrote:
>> Compiling 2.6.19-rc6 with gcc version 4.1.0 (SUSE Linux),
>> wait_hpet_tick is optimized away to a never ending loop and the kernel
>> hangs on boot in timer setup.
>> 
>> 0000001a <wait_hpet_tick>:
>>   1a:   55                      push   %ebp
>>   1b:   89 e5                   mov    %esp,%ebp
>>   1d:   eb fe                   jmp    1d <wait_hpet_tick+0x3>
>> 
>> This is not a problem with gcc 3.3.5.  Adding barrier() calls to
>> wait_hpet_tick does not help, making the variables volatile does.
>> 
>> Signed-off-by: Keith Owens <kaos@....com.au>
>> 
>> ---
>>  arch/i386/kernel/time_hpet.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> Index: linux-2.6/arch/i386/kernel/time_hpet.c
>> ===================================================================
>> --- linux-2.6.orig/arch/i386/kernel/time_hpet.c
>> +++ linux-2.6/arch/i386/kernel/time_hpet.c
>> @@ -51,7 +51,7 @@ static void hpet_writel(unsigned long d,
>>   */
>>  static void __devinit wait_hpet_tick(void)
>>  {
>> -	unsigned int start_cmp_val, end_cmp_val;
>> +	unsigned volatile int start_cmp_val, end_cmp_val;
>>  
>>  	start_cmp_val = hpet_readl(HPET_T0_CMP);
>>  	do {
>
>When you examine the inlined functions involved, this looks an awful lot
>like http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22278
>
>Perhaps SUSE should fix their gcc instead of working around compiler
>problems in the kernel?

Firstly, the fix for 22278 is included in gcc 4.1.0.

Secondly, I believe that this is a separate problem from bug 22278.
hpet_readl() is correctly using volatile internally, but its result is
being assigned to a pair of normal integers (not declared as volatile).
In the context of wait_hpet_tick, all the variables are unqualified so
gcc is allowed to optimize the comparison away.

The same problem may exist in other parts of arch/i386/kernel/time_hpet.c,
where the return value from hpet_readl() is assigned to a normal
variable.  Nothing in the C standard says that those unqualified
variables should be magically treated as volatile, just because the
original code that extracted the value used volatile.  IOW, time_hpet.c
needs to declare any variables that hold the result of hpet_readl() as
being volatile variables.

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