[3.16.y-ckt stable] Patch "rcu: Correctly handle non-empty Tiny RCU callback list with none ready" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    rcu: Correctly handle non-empty Tiny RCU callback list with none ready

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt15.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 61ee404d0e11f426b1b67817d05dc177d3610ae2 Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
Date: Mon, 11 May 2015 11:13:05 -0700
Subject: rcu: Correctly handle non-empty Tiny RCU callback list with none
 ready

commit 6e91f8cb138625be96070b778d9ba71ce520ea7e upstream.

If, at the time __rcu_process_callbacks() is invoked,  there are callbacks
in Tiny RCU's callback list, but none of them are ready to be invoked,
the current list-management code will knit the non-ready callbacks out
of the list.  This can result in hangs and possibly worse.  This commit
therefore inserts a check for there being no callbacks that can be
invoked immediately.

This bug is unlikely to occur -- you have to get a new callback between
the time rcu_sched_qs() or rcu_bh_qs() was called, but before we get to
__rcu_process_callbacks().  It was detected by the addition of RCU-bh
testing to rcutorture, which in turn was instigated by Iftekhar Ahmed's
mutation testing.  Although this bug was made much more likely by
915e8a4fe45e (rcu: Remove fastpath from __rcu_process_callbacks()), this
did not cause the bug, but rather made it much more probable.   That
said, it takes more than 40 hours of rcutorture testing, on average,
for this bug to appear, so this fix cannot be considered an emergency.

Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh at joshtriplett.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/rcu/tiny.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index d9efcc13008c..90fdef66aa4a 100644
--- a/kernel/rcu/tiny.c
+++ b/kernel/rcu/tiny.c
@@ -280,6 +280,11 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)

 	/* Move the ready-to-invoke callbacks to a local list. */
 	local_irq_save(flags);
+	if (rcp->donetail == &rcp->rcucblist) {
+		/* No callbacks ready, so just leave. */
+		local_irq_restore(flags);
+		return;
+	}
 	RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, rcp->qlen, -1));
 	list = rcp->rcucblist;
 	rcp->rcucblist = *rcp->donetail;