linux-kernel - [PATCH tip/core/rcu 5/6] rcu: Add comment giving debug strategy for double call

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Message-Id: <1512157320-23205-5-git-send-email-paulmck@linux.vnet.ibm.com>
Date:   Fri,  1 Dec 2017 11:41:59 -0800
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 5/6] rcu: Add comment giving debug strategy for double call_rcu()

The following statement has for some reason proven non-intuitive:

	WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));

This commit therefore adds a comment that states that this warning
usually triggers in response to a double call_rcu(), which is sort
of like a double free.  The comment also suggests building with
CONFIG_DEBUG_OBJECTS_RCU_HEAD=y to track down the double call_rcu().

Reported-by: David Howells <dhowells@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 70c48c279323..a63b14ba3a44 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2789,6 +2789,11 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 		rdp->n_force_qs_snap = rsp->n_force_qs;
 	} else if (count < rdp->qlen_last_fqs_check - qhimark)
 		rdp->qlen_last_fqs_check = count;
+
+	/*
+	 * The following usually indicates a double call_rcu().  To track
+	 * this down, try building with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y.
+	 */
 	WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));
 
 	local_irq_restore(flags);
-- 
2.5.2