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SubjectRe: [patch 2.6.19-rc6] Stop gcc 4.1.0 optimizing wait_hpet_tick away
Date
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@ocs.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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