rcu: Consolidate last open-coded expedited memory barrier (704dd435) · Commits · 戴 / test

One of the requirements on RCU grace periods is that if there is a causal chain of operations that starts after one grace period and ends before another grace period, then the two grace periods must be serialized. There has been (and might still be) code that relies on this, for example, certain types of reference-counting code that does a call_rcu() within an RCU callback function. This requirement is why there is an smp_mb() at the end of both synchronize_sched_expedited() and synchronize_rcu_expedited(). However, this is the only smp_mb() in these functions, so it would be nicer to consolidate it into rcu_exp_gp_seq_end(). This commit does just that. Signed-off-by:

Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -3299,6 +3299,7 @@ static void rcu_exp_gp_seq_start(struct rcu_state *rsp)
		static void rcu_exp_gp_seq_end(struct rcu_state *rsp)
		{
		rcu_seq_end(&rsp->expedited_sequence);
		smp_mb(); /* Ensure that consecutive grace periods serialize. */
		}
		static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp)
		{
		@@ -3431,7 +3432,6 @@ void synchronize_sched_expedited(void)

		rcu_exp_gp_seq_end(rsp);
		mutex_unlock(&rnp->exp_funnel_mutex);
		smp_mb(); /* ensure subsequent action seen after grace period. */

		put_online_cpus();
		}

kernel/rcu/tree_plugin.h

+0 −1

Original line number	Diff line number	Diff line
		@@ -735,7 +735,6 @@ void synchronize_rcu_expedited(void)
		/* Clean up and exit. */
		rcu_exp_gp_seq_end(rsp);
		mutex_unlock(&rnp_unlock->exp_funnel_mutex);
		smp_mb(); /* ensure subsequent action seen after grace period. */
		}
		EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

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