Patch "rcu: Correctly handle non-empty Tiny RCU callback list with none ready" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
rcu-correctly-handle-non-empty-tiny-rcu-callback-list-with-none-ready.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Paul E. McKenney" <[email protected]>
Date: Mon, 11 May 2015 11:13:05 -0700
Subject: rcu: Correctly handle non-empty Tiny RCU callback list with none ready

From: "Paul E. McKenney" <[email protected]>

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.

---
 kernel/rcu/tiny.c |    5 +++++
 1 file changed, 5 insertions(+)

@@ -284,6 +284,11 @@ static void __rcu_process_callbacks(stru
 
 	/* 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;

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/rcu-correctly-handle-non-empty-tiny-rcu-callback-list-with-none-ready.patch