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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@xxxxxxxxx > To: buyit@xxxxxxx > CC: kernelnewbies@xxxxxxxxxxxxxxxxx > > 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@xxxxxxxxx>: > > 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@xxxxxxx>: > >> 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@xxxxxxxxx > >>> To: Kernelnewbies@xxxxxxxxxxxxxxxxx > >> > >>> > >>> 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 > >>> > >>> _______________________________________________ > >>> Kernelnewbies mailing list > >>> Kernelnewbies@xxxxxxxxxxxxxxxxx > >>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > _______________________________________________ > Kernelnewbies mailing list > Kernelnewbies@xxxxxxxxxxxxxxxxx > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies |
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