rcu: Avoid modifying mask_ofl_ipi in sync_rcu_exp_select_node_cpus()

The "mask_ofl_ipi" is used to track which CPUs get IPIed, however
in the IPI sending loop, "mask_ofl_ipi" along with another variable
"mask_ofl_test" might also get modified to record which CPUs' quiesent
states must be reported by the sync_rcu_exp_select_node_cpus() at
the end of sync_rcu_exp_select_node_cpus().  This overlap of roles
can be confusing, so this patch cleans things a little by using
"mask_ofl_ipi" solely for determining which CPUs must be IPIed  and
"mask_ofl_test" for solely determining on behalf of  which CPUs
sync_rcu_exp_select_node_cpus() must report a quiscent state.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Marco Elver <elver@google.com>

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 69c5aa6..6a6f328 100644 (file)
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -387,10 +387,10 @@ retry_ipi:
                }
                ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
                put_cpu();
-               if (!ret) {
-                       mask_ofl_ipi &= ~mask;
+               /* The CPU will report the QS in response to the IPI. */
+               if (!ret)
                        continue;
-               }
+
                /* Failed, raced with CPU hotplug operation. */
                raw_spin_lock_irqsave_rcu_node(rnp, flags);
                if ((rnp->qsmaskinitnext & mask) &&
@@ -401,13 +401,12 @@ retry_ipi:
                        schedule_timeout_uninterruptible(1);
                        goto retry_ipi;
                }
-               /* CPU really is offline, so we can ignore it. */
-               if (!(rnp->expmask & mask))
-                       mask_ofl_ipi &= ~mask;
+               /* CPU really is offline, so we must report its QS. */
+               if (rnp->expmask & mask)
+                       mask_ofl_test |= mask;
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
        }
        /* Report quiescent states for those that went offline. */
-       mask_ofl_test |= mask_ofl_ipi;
        if (mask_ofl_test)
                rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
 }