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SubjectRe: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context
On 12/23, Srivatsa S. Bhat wrote:
>
> On 12/20/2012 07:12 PM, Oleg Nesterov wrote:
> >
> > We need mb() + rmb(). Plust cli/sti unless this arch has optimized
> > this_cpu_add() like x86 (as you pointed out).
> >
>
> Hey, IIUC, we actually don't need mb() in the reader!! Just an rmb() will do.

Well. I don't think so. But when it comes to the barriers I am never sure
until Paul confirms my understanding ;)

> #define reader_nested_percpu() \
> (__this_cpu_read(reader_percpu_refcnt) & READER_REFCNT_MASK)
>
> #define writer_active() \
> (__this_cpu_read(writer_signal))
>
>
> #define READER_PRESENT (1UL << 16)
> #define READER_REFCNT_MASK (READER_PRESENT - 1)
>
> void get_online_cpus_atomic(void)
> {
> preempt_disable();
>
> /*
> * First and foremost, make your presence known to the writer.
> */
> this_cpu_add(reader_percpu_refcnt, READER_PRESENT);
>
> /*
> * If we are already using per-cpu refcounts, it is not safe to switch
> * the synchronization scheme. So continue using the refcounts.
> */
> if (reader_nested_percpu()) {
> this_cpu_inc(reader_percpu_refcnt);
> } else {
> smp_rmb();
> if (unlikely(writer_active())) {
> ... //take hotplug_rwlock
> }
> }
>
> ...
>
> /* Prevent reordering of any subsequent reads of cpu_online_mask. */
> smp_rmb();
> }
>
> The smp_rmb() before writer_active() ensures that LOAD(writer_signal) follows
> LOAD(reader_percpu_refcnt) (at the 'if' condition). And in turn, that load is
> automatically going to follow the STORE(reader_percpu_refcnt)

But why this STORE should be visible on another CPU before we LOAD(writer_signal)?

Lets discuss the simple and artificial example. Suppose we have

int X, Y;

int func(void)
{
X = 1; // suppose that nobody else can change it
mb();
return Y;
}

Now you are saying that we can change it and avoid the costly mb():

int func(void)
{
X = 1;

if (X != 1)
BUG();

rmb();
return Y;
}

I doubt. rmb() can only guarantee that the preceding LOAD's should be
completed. Without mb() it is possible that this CPU won't write X to
memory at all.

Oleg.



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