From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
This patch has been added to the 3.18 stable tree. If you have any
objections, please let us know.
===============
[ Upstream commit 6e91f8cb138625be96070b778d9ba71ce520ea7e ]
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@xxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
---
kernel/rcu/tiny.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index c0623fc..da80f2f 100644
--- a/kernel/rcu/tiny.c
+++ b/kernel/rcu/tiny.c
@@ -282,6 +282,11 @@ 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;
--
2.1.0
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