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Subject[PATCH tip/core/rcu 02/10] rcu: Avoid modifying mask_ofl_ipi in sync_rcu_exp_select_node_cpus()
Date
From: Boqun Feng <boqun.feng@gmail.com>

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>
---
kernel/rcu/tree_exp.h | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 69c5aa6..6a6f328 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -387,10 +387,10 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
}
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 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
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);
}
--
2.9.5
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