Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

From: Paul E. McKenney
Date: Tue Oct 23 2012 - 16:36:49 EST

On Tue, Oct 23, 2012 at 08:41:23PM +0200, Oleg Nesterov wrote:
> On 10/23, Paul E. McKenney wrote:
> >
> > * Note that this guarantee implies a further memory-ordering guarantee.
> > * On systems with more than one CPU, when synchronize_sched() returns,
> > * each CPU is guaranteed to have executed a full memory barrier since
> > * the end of its last RCU read-side critical section
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Ah wait... I misread this comment.

And I miswrote it. It should say "since the end of its last RCU-sched
read-side critical section." So, for example, RCU-sched need not force
a CPU that is idle, offline, or (eventually) executing in user mode to
execute a memory barrier. Fixed this.

> But this patch needs more? Or I misunderstood. There is no RCU unlock
> in percpu_up_read().
> IOW. Suppose the code does
> percpu_down_read();
> percpu_up_read();
> Withoit mb() the load above can be reordered with this_cpu_dec() in
> percpu_up_read().
> However, we do not care if we can guarantee that the next
> percpu_down_write() can not return (iow, the next "write" section can
> not start) until this load is complete.
> And I _think_ that another synchronize_sched() in percpu_down_write()
> added by this patch should work.
> But, "since the end of its last RCU read-side critical section"
> does not look enough.
> Or I misundersood you/Mikulas/both ?

I clearly need to look more carefully at Mikulas's code...

Thanx, Paul

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