Commit 55911a9f authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Paul E. McKenney
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rcu: Remove full memory barrier on RCU stall printout 



RCU stall printout fetches the EQS state of a CPU with a preceding full
memory barrier. However there is nothing to order this read against at
this debugging stage. It is inherently racy when performed remotely.

Do a plain read instead.

This was the last user of rcu_dynticks_snap().

Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Reviewed-by: default avatarBoqun Feng <boqun.feng@gmail.com>
Reviewed-by: default avatarNeeraj Upadhyay <neeraj.upadhyay@kernel.org>
parent e7a3c8ea

kernel/rcu/tree.c

+0 −10
Original line number Diff line number Diff line
@@ -295,16 +295,6 @@ static void rcu_dynticks_eqs_online(void) 
	ct_state_inc(RCU_DYNTICKS_IDX);

}



/*

 * Snapshot the ->dynticks counter with full ordering so as to allow

 * stable comparison of this counter with past and future snapshots.

 */

static int rcu_dynticks_snap(int cpu)

{

	smp_mb();  // Fundamental RCU ordering guarantee.

	return ct_dynticks_cpu_acquire(cpu);

}



/*

 * Return true if the snapshot returned from rcu_dynticks_snap()

 * indicates that RCU is in an extended quiescent state.

kernel/rcu/tree_stall.h

+2 −2
Original line number Diff line number Diff line
@@ -501,7 +501,7 @@ static void print_cpu_stall_info(int cpu) 
	}

	delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);

	falsepositive = rcu_is_gp_kthread_starving(NULL) &&

			rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu));

			rcu_dynticks_in_eqs(ct_dynticks_cpu(cpu));

	rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);

	if (rcuc_starved)

		// Print signed value, as negative values indicate a probable bug.
@@ -515,7 +515,7 @@ static void print_cpu_stall_info(int cpu) 
			rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :

				"!."[!delta],

	       ticks_value, ticks_title,

	       rcu_dynticks_snap(cpu) & 0xffff,

	       ct_dynticks_cpu(cpu) & 0xffff,

	       ct_dynticks_nesting_cpu(cpu), ct_dynticks_nmi_nesting_cpu(cpu),

	       rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),

	       data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,