Messages in this thread | | | Date | Mon, 17 Feb 2020 13:23:02 -0500 | From | Joel Fernandes <> | Subject | Re: [PATCH tip/core/rcu 1/3] rcu-tasks: *_ONCE() for rcu_tasks_cbs_head |
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On Mon, Feb 17, 2020 at 01:38:51PM +0100, Peter Zijlstra wrote: > On Fri, Feb 14, 2020 at 04:25:18PM -0800, paulmck@kernel.org wrote: > > From: "Paul E. McKenney" <paulmck@kernel.org> > > > > The RCU tasks list of callbacks, rcu_tasks_cbs_head, is sampled locklessly > > by rcu_tasks_kthread() when waiting for work to do. This commit therefore > > applies READ_ONCE() to that lockless sampling and WRITE_ONCE() to the > > single potential store outside of rcu_tasks_kthread. > > > > This data race was reported by KCSAN. Not appropriate for backporting > > due to failure being unlikely. > > What failure is possible here? AFAICT this is (again) one of them > load-complare-against-constant-discard patterns that are impossible to > mess up.
You mean that because we are only testing for NULL, so load/store tearing of rcu_tasks_cbs_head is not an issue right?
I agree. Even with invented stores, worst case we have a false-wakeup and go right back to sleep. Or, we read a partial rcu_tasks_cbs_head, and then go acquire the lock and read the whole thing correctly under lock.
I wonder if we can teach KCSAN to actually ignore this kind of situation so we don't need to employ READ_ONCE() for no reason. Basically ask it to not bother if the read was only NULL-testing. +Marco since it is KCSAN related.
thanks,
- Joel
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > > --- > > kernel/rcu/update.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > index 6c4b862..a27df76 100644 > > --- a/kernel/rcu/update.c > > +++ b/kernel/rcu/update.c > > @@ -528,7 +528,7 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func) > > rhp->func = func; > > raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); > > needwake = !rcu_tasks_cbs_head; > > - *rcu_tasks_cbs_tail = rhp; > > + WRITE_ONCE(*rcu_tasks_cbs_tail, rhp); > > rcu_tasks_cbs_tail = &rhp->next; > > raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); > > /* We can't create the thread unless interrupts are enabled. */ > > @@ -658,7 +658,7 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > /* If there were none, wait a bit and start over. */ > > if (!list) { > > wait_event_interruptible(rcu_tasks_cbs_wq, > > - rcu_tasks_cbs_head); > > + READ_ONCE(rcu_tasks_cbs_head)); > > if (!rcu_tasks_cbs_head) { > > WARN_ON(signal_pending(current)); > > schedule_timeout_interruptible(HZ/10); > > -- > > 2.9.5 > >
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