On Tue, May 12, 2015 at 03:49:13PM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> > > 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> Ouch, subtle. Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx> > kernel/rcu/tiny.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c > index a501b4ab9b1c..591af0cb7b9f 100644 > --- a/kernel/rcu/tiny.c > +++ b/kernel/rcu/tiny.c > @@ -137,6 +137,11 @@ 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; > -- > 1.8.1.5 > -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html