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

From: Oleg Nesterov
Date: Tue Oct 23 2012 - 14:40:27 EST

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.

But this patch needs more? Or I misunderstood. There is no RCU unlock
in percpu_up_read().

IOW. Suppose the code does


Withoit mb() the load above can be reordered with this_cpu_dec() in

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 ?


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