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Date:   Tue, 21 Nov 2023 21:30:49 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Ankur Arora <ankur.a.arora@oracle.com>,
        linux-kernel@vger.kernel.org, tglx@linutronix.de,
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Subject: Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT_RCU=n
Message-ID: <20231121203049.GN8262@noisy.programming.kicks-ass.net>
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On Tue, Nov 21, 2023 at 11:25:18AM -0800, Paul E. McKenney wrote:
> #define preempt_enable() \
> do { \
> 	barrier(); \
> 	if (!IS_ENABLED(CONFIG_PREEMPT_RCU) && raw_cpu_read(rcu_data.rcu_urgent_qs) && \
> 	    (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK | HARDIRQ_MASK | NMI_MASK) == PREEMPT_OFFSET) &&
> 	    !irqs_disabled()) \
> 		rcu_all_qs(); \
> 	if (unlikely(preempt_count_dec_and_test())) { \
> 		__preempt_schedule(); \
> 	} \
> } while (0)

Aaaaahhh, please no. We spend so much time reducing preempt_enable() to
the minimal thing it is today, this will make it blow up into something
giant again.