Memory barrier

[卜弋天]

Hi :
 
      if you write code as below:
 
golbal int in1=0,int2=0;
 
cpu1:                     cpu2:
 
int1 = 1;                 b= int2;
smp_wmb()
int2 = 2;                 a = in1;
 
cpu2 may get the result: b==2 & a==0 , which means although cpu1 set int1=1 before int2=2, there is no garentee for cpu2 to perceive int1 before int2.
you must add smp_rmb() inside cpu2 to prevent this.
 
two cpus must cooperate to acheive the sequence memory order. 
 
 

 

> Date: Fri, 9 Dec 2011 14:14:37 +0530
> Subject: Re: Memory barrier
> From: trisha1march at gmail.com
> To: buyit at live.cn
> CC: kernelnewbies at kernelnewbies.org
> 
> I will add more info here:
> smp_mb()
> Similar to mb(), but only guarantees ordering between cores/processors
> within an SMP system. All memory accesses before the smp_mb() will be
> visible to all cores within the SMP system before any accesses after
> the smp_mb().
> smp_rmb()
> Like smp_mb(), but only guarantees ordering between read accesses.
> smp_wmb()
> Like smp_mb(), but only guarantees ordering between write accesses.
> 
> So these made me total confuse .
> 
> Thanks
> 
> 2011/12/9 trisha yad <trisha1march at gmail.com>:
> > Thanks,
> >
> > I got bit confuse with below statement:
> > This is from paper Memory access ordering Part 2
> > SMP conditional barriers
> > The SMP conditional barriers are used to ensure a consistent view of
> > memory between different cores within a cache coherent SMP system.
> > When compiling a kernel without CONFIG_SMP, all SMP barriers are
> > converted into plain compiler barriers.
> >
> > 2011/12/9 卜弋天 <buyit at live.cn>:
> >> Hi :
> >>
> >> memory barriers can not make order on other cpus, only the current
> >> cpu's order will be promised.
> >>
> >>
> >>
> >>> Date: Fri, 9 Dec 2011 12:54:40 +0530
> >>> Subject: Memory barrier
> >>> From: trisha1march at gmail.com
> >>> To: Kernelnewbies at kernelnewbies.org
> >>
> >>>
> >>> Hi All,
> >>>
> >>> I need small clarification on memory barrier.
> >>> #define smp_mb() mb()
> >>> #define smp_rmb() rmb()
> >>> #define smp_wmb() wmb()
> >>> In case of SMP:
> >>> is smp_mb() or smp_rmb() make order on current CPU or all cpu's
> >>>
> >>> Thanks
> >>>
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