linux-kernel - [tip: core/rcu] rcu: Avoid modifying mask_ofl_ipi in sync_rcu_exp_select_node

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Date:   Sat, 25 Jan 2020 10:42:58 -0000
From:   "tip-bot2 for Boqun Feng" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Marco Elver <elver@...gle.com>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: core/rcu] rcu: Avoid modifying mask_ofl_ipi in
 sync_rcu_exp_select_node_cpus()

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     9f08cf088676c12a5b53bd5a29cf04f00c787b5d
Gitweb:        https://git.kernel.org/tip/9f08cf088676c12a5b53bd5a29cf04f00c787b5d
Author:        Boqun Feng <boqun.feng@...il.com>
AuthorDate:    Tue, 08 Oct 2019 13:01:40 +08:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Mon, 09 Dec 2019 12:24:56 -08:00

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@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Acked-by: Marco Elver <elver@...gle.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 @@ 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);
 }