Re: question about RCU dynticks_nesting

[Peter Zijlstra]

On Mon, May 04, 2015 at 03:00:44PM -0400, Rik van Riel wrote:
 In case of the non-preemptible RCU, we could easily also
> increase current->rcu_read_lock_nesting at the same time
> we increase the preempt counter, and use that as the
> indicator to test whether the cpu is in an extended
> rcu quiescent state. That way there would be no extra
> overhead at syscall entry or exit at all. The trick
> would be getting the preempt count and the rcu read
> lock nesting count in the same cache line for each task.

Can't do that. Remember, on x86 we have per-cpu preempt count, and your
rcu_read_lock_nesting is per task.
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