Messages in this thread | | | Date | Tue, 23 Oct 2012 20:41:23 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers |
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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
percpu_down_read(); x = PROTECTED_BY_THIS_RW_SEM; 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 ?
Oleg.
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