Synchronize_rcu() and synchronize_sched() in _cpu_down path

[Vignesh Radhakrishnan]

Hi,

I had a query regarding calling synchronize_rcu and synchronize_sched in
_cpu_down path :
Here is the snippet below :

        /*
         * By now we've cleared cpu_active_mask, wait for all
preempt-disabled
         * and RCU users of this state to go away such that all new such
users
         * will observe it.
         *
         * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu()
might
         * not imply sync_sched(), so explicitly call both.
         *
         * Do sync before park smpboot threads to take care the rcu boost
case.
         */
#ifdef CONFIG_PREEMPT
        synchronize_sched();
#endif
        synchronize_rcu();

My query is that :
During cpu_down path, we do migrate tasks in this CPU to another CPU

 As per LWN article here - http://lwn.net/Articles/253651/ :

*For example, suppose that a task calls rcu_read_lock() on one CPU, is
preempted, resumes on another CPU, and then calls rcu_read_unlock(). The
first CPU's counter will then be +1 and the second CPU's counter will be
-1, however, they will still sum to zero. Regardless of possible
preemption, when the sum of the old counter elements does go to zero, it is
safe to move to the next grace-period stage, as shown on the right-hand
side of the above figure. *

>From what i understand : Since we migrate the tasks to another CPU, the
read side critical sections should get completed from other CPU on which
this task is going to run.
Therefore why call synchronize_rcu()/synchronize_sched()  and wait in the
cpu_down path while there is no write operations happening here in this
code?

Thanks and regards,
Vignesh Radhakrishnan
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