Commit 46103fe0 authored Jan 18, 2023 by Zqiang Committed by Joel Fernandes (Google) Apr 05, 2023

rcu: Remove never-set needwake assignment from rcu_report_qs_rdp()



The rcu_accelerate_cbs() function is invoked by rcu_report_qs_rdp()
only if there is a grace period in progress that is still blocked
by at least one CPU on this rcu_node structure.  This means that
rcu_accelerate_cbs() should never return the value true, and thus that
this function should never set the needwake variable and in turn never
invoke rcu_gp_kthread_wake().

This commit therefore removes the needwake variable and the invocation
of rcu_gp_kthread_wake() in favor of a WARN_ON_ONCE() on the call to
rcu_accelerate_cbs().  The purpose of this new WARN_ON_ONCE() is to
detect situations where the system's opinion differs from ours.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

parent 2450b78e

kernel/rcu/tree.c

+6 −4

Original line number	Diff line number	Diff line
		@@ -1955,7 +1955,6 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
		{
		unsigned long flags;
		unsigned long mask;
		bool needwake = false;
		bool needacc = false;
		struct rcu_node *rnp;

		@@ -1987,7 +1986,12 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
		* NOCB kthreads have their own way to deal with that...
		*/
		if (!rcu_rdp_is_offloaded(rdp)) {
		needwake = rcu_accelerate_cbs(rnp, rdp);
		/*
		* The current GP has not yet ended, so it
		* should not be possible for rcu_accelerate_cbs()
		* to return true. So complain, but don't awaken.
		*/
		WARN_ON_ONCE(rcu_accelerate_cbs(rnp, rdp));
		} else if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) {
		/*
		* ...but NOCB kthreads may miss or delay callbacks acceleration
		@@ -1999,8 +2003,6 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
		rcu_disable_urgency_upon_qs(rdp);
		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
		/* ^^^ Released rnp->lock */
		if (needwake)
		rcu_gp_kthread_wake();

		if (needacc) {
		rcu_nocb_lock_irqsave(rdp, flags);