At Linux Kernel 2.6.15 kernel/timer.c , memory barrier is used.
I can't understand why memory barrier must be used.
/* comment */
asmlinkage long sys_getppid(void)
{
int pid;
struct task_struct *me = current;
struct task_struct *parent;
parent = me->group_leader->real_parent;
for (;;) {
pid = parent->tgid;
#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
{
struct task_struct *old = parent;
/*
* Make sure we read the pid before re-reading the
* parent pointer :
*/
smp_rmb(); <----- Why ?
parent = me->group_leader->real_parent;
if (old != parent)
continue;
}
#endif
break;
}
return pid ;
}
I read code comment also, But I can't find any reason.
How pid variable is read after parent pointer re-reading ?
Please, let me know ...
Thank you for your help..
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