Hi Paul,
Thanks a lot for the correction. Thanks's right; we need to care about
other achitectures :-)
So barrier() is basically empty: define barrier() __asm__
__volatile__("": : :"memory") . I'm curious about what on the earth it
does? We do really use it in counttorture.h. Any reference to
resources is welcome!
Even if we agree with smp_mb(), count_stat_eventual.c seems not 100%
correct for me. Please check the following patch. It seems to me the
barrier instruction (smp_mb) in function count_cleanup is to make RAW
correct. So it should be inserted between the write and read
instructions. Is that right?
index 75a0ca9..32f7e80 100644
--- a/CodeSamples/count/count_stat_eventual.c
+++ b/CodeSamples/count/count_stat_eventual.c
@@ -67,9 +67,9 @@ void count_init(void)
void count_cleanup(void)
{
stopflag = 1;
+ smp_mb();
while (stopflag < 3)
poll(NULL, 0, 1);
- smp_mb();
}
#include "counttorture.h"
--
2.7.4
Thanks,
--Jason
On Tue, May 9, 2017 at 10:47 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Tue, May 09, 2017 at 01:08:31PM +0800, Junchang Wang wrote:
>> count_stat_eventual.c uses a single global variable (stopflag) to synchronize
>> two separate threads (main and eventual) probably running on two different CPU
>> cores. My understanding is that for this model, memory fence instruction
>> (mfence) which typically are expensive can be avoided, only if we make sure (1)
>> that compiler neither reorder the code around nor cache the variable, and (2)
>> that the CPU core running code precedes in program order.
>>
>> We use keyword ``volatile'' to prevent compilers from reordering the code and
>> caching the variable, and instruction barrier() to force CPU core running the
>> code to obey program order.
>
> Please take another look at the barrier() macro. It does not emit any
> instructions, so cannot force the CPU to do much of anything. Keep in
> mind that this code needs to run on weakly ordered systems, not just x86.
>
> Thanx, Paul
>
>> Signed-off-by: Junchang Wang <junchangwang@xxxxxxxxx>
>> ---
>> CodeSamples/count/count_stat_eventual.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/CodeSamples/count/count_stat_eventual.c b/CodeSamples/count/count_stat_eventual.c
>> index 059ab8b..5ffc4aa 100644
>> --- a/CodeSamples/count/count_stat_eventual.c
>> +++ b/CodeSamples/count/count_stat_eventual.c
>> @@ -23,7 +23,7 @@
>>
>> DEFINE_PER_THREAD(unsigned long, counter);
>> unsigned long global_count;
>> -int stopflag;
>> +volatile int stopflag;
>>
>> void inc_count(void)
>> {
>> @@ -47,7 +47,7 @@ void *eventual(void *arg)
>> ACCESS_ONCE(global_count) = sum;
>> poll(NULL, 0, 1);
>> if (stopflag) {
>> - smp_mb();
>> + barrier();
>> stopflag++;
>> }
>> }
>> @@ -67,9 +67,9 @@ void count_init(void)
>> void count_cleanup(void)
>> {
>> stopflag = 1;
>> + barrier();
>> while (stopflag < 3)
>> poll(NULL, 0, 1);
>> - smp_mb();
>> }
>>
>> #include "counttorture.h"
>> --
>> 2.7.4
>>
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