Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     linux-kernel@vger.kernel.org
Cc:     mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        fweisbec@gmail.com, oleg@redhat.com, joel.opensrc@gmail.com,
        torvalds@linux-foundation.org, npiggin@gmail.com,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 06/21] rcu: Avoid losing ->need_future_gp[] values due to GP start/end races
Date:   Sun, 22 Apr 2018 20:03:29 -0700
In-Reply-To: <20180423030258.GA23370@linux.vnet.ibm.com>
References: <20180423030258.GA23370@linux.vnet.ibm.com>
Message-Id: <1524452624-27589-6-git-send-email-paulmck@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

The rcu_cbs_completed() function provides the value of ->completed
at which new callbacks can safely be invoked.  This is recorded in
two-element ->need_future_gp[] arrays in the rcu_node structure, and
the elements of these arrays corresponding to the just-completed grace
period are zeroed at the end of that grace period.  However, the
rcu_cbs_completed() function can return the current ->completed value
plus either one or two, so it is possible for the corresponding
->need_future_gp[] entry to be cleared just after it was set, thus
losing a request for a future grace period.

This commit avoids this race by expanding ->need_future_gp[] to four
elements.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index bd1103763551..952cd0c223fe 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -150,8 +150,7 @@ struct rcu_node {
 	struct swait_queue_head nocb_gp_wq[2];
 				/* Place for rcu_nocb_kthread() to wait GP. */
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
-	int need_future_gp[2];
-				/* Counts of upcoming no-CB GP requests. */
+	int need_future_gp[4];	/* Counts of upcoming no-CB GP requests. */
 	raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
 
 	spinlock_t exp_lock ____cacheline_internodealigned_in_smp;
-- 
2.5.2