linux-kernel - [PATCH tip/core/rcu 12/12] rcu: Move callback-list warning to irq-disable region

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Date:   Mon, 24 Jul 2017 14:51:56 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 12/12] rcu: Move callback-list warning to irq-disable region

After adopting callbacks from a newly offlined CPU, the adopting CPU
checks to make sure that its callback list's count is zero only if the
list has no callbacks and vice versa.  Unfortunately, it does so after
enabling interrupts, which means that false positives are possible due to
interrupt handlers invoking call_rcu().  Although these false positives
are improbable, rcutorture did make it happen once.

This commit therefore moves this check to an irq-disabled region of code,
thus suppressing the false positive.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d330c17c8df4..4b03bddbca3c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3869,14 +3869,14 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
 	rcu_advance_cbs(rsp, rnp_root, rdp); /* Leverage recent GPs. */
 	rcu_advance_cbs(rsp, rnp_root, my_rdp); /* Assign GP to pending CBs. */
 	rcu_segcblist_merge(&my_rdp->cblist, &rdp->cblist);
+	WARN_ON_ONCE(rcu_segcblist_empty(&my_rdp->cblist) !=
+		     !rcu_segcblist_n_cbs(&my_rdp->cblist));
 	raw_spin_unlock_irqrestore_rcu_node(rnp_root, flags);
 	WARN_ONCE(rcu_segcblist_n_cbs(&rdp->cblist) != 0 ||
 		  !rcu_segcblist_empty(&rdp->cblist),
 		  "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, 1stCB=%p\n",
 		  cpu, rcu_segcblist_n_cbs(&rdp->cblist),
 		  rcu_segcblist_first_cb(&rdp->cblist));
-	WARN_ON_ONCE(rcu_segcblist_empty(&my_rdp->cblist) !=
-		     !rcu_segcblist_n_cbs(&my_rdp->cblist));
 }
 
 /*
-- 
2.5.2