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Date: Tue, 5 May 2015 12:48:34 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Rik van Riel <riel@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Ingo Molnar <mingo@kernel.org>,
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        "Paul E. McKenney" <paulmck@us.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: question about RCU dynticks_nesting
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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.