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Kernels built with CONFIG_PREEMPTION=n and CONFIG_PREEMPT_COUNT=y maintain preempt_count() state. Because such kernels map __rcu_read_lock() and __rcu_read_unlock() to preempt_disable() and preempt_enable(), respectively, this allows the expedited grace period's !CONFIG_PREEMPT_RCU version of the rcu_exp_handler() IPI handler function to use preempt_count() to detect quiescent states. This preempt_count() usage might seem to risk failures due to use of implicit RCU readers in portions of the kernel under #ifndef CONFIG_PREEMPTION, except that rcu_core() already disallows such implicit RCU readers. The moral of this story is that you must use explicit read-side markings such as rcu_read_lock() or preempt_disable() even if the code knows that this kernel does not support preemption. This commit therefore adds a preempt_count()-based check for a quiescent state in the !CONFIG_PREEMPT_RCU version of the rcu_exp_handler() function for kernels built with CONFIG_PREEMPT_COUNT=y, reporting an immediate quiescent state when the interrupted code had both preemption and softirqs enabled. This change results in about a 2% reduction in expedited grace-period latency in kernels built with both CONFIG_PREEMPT_RCU=n and CONFIG_PREEMPT_COUNT=y. Signed-off-by: Zqiang <qiang1.zhang@intel.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Link: https://lore.kernel.org/all/20220622103549.2840087-1-qiang1.zhang@intel.com/