Re: [PATCH RT 1/2] tasklet: Address a race resulting in double-enqueue

[Tom Zanussi]

Hi Ramon,

On Tue, 2020-06-09 at 20:14 +0300, Ramon Fried wrote:
> 
> On June 9, 2020 8:10:43 PM GMT+03:00, Sebastian Andrzej Siewior <
> bigeasy@xxxxxxxxxxxxx> wrote:
> > On 2020-06-09 20:07:06 [+0300], Ramon Fried wrote:
> > > Indeed
> > >  I'm truly sorry, I thought our crash kernel is configured as RT
> > > as
> > 
> > well. 
> > > so, as I understand, if I build the RT kernel without preempt
> > > enabled
> > 
> > I can hit this bug? 
> > 
> > Don't worry, I should have been better with the details in the log.
> > 
> > So you should _always_ hit the warning/bug if you compile a kernel
> > without SMP and RT. If you enable one of these then everything
> > should
> > be
> > fine.
> 
> Would there be a fix for that? 

I haven't tested the fix yet, but can you try the below patch and see
if it fixes your broken case?

[PATCH] tasklet: Fix UP case for tasklet CHAINED state

commit 62d0a2a30cd0 (tasklet: Address a race resulting in
double-enqueue) addresses a problem that can result in a tasklet being
enqueued on two cpus at the same time by combining the RUN flag with a
new CHAINED flag, and relies on the combination to be present in order
to zero it out, which can never happen on (!SMP and !PREEMPT_RT_FULL)
because the RUN flag is SMP/PREEMPT_RT_FULL-only.

So make sure the above commit is only applied for the SMP ||
PREEMPT_RT_FULL case.

Signed-off-by: Tom Zanussi <zanussi@xxxxxxxxxx>
---
 kernel/softirq.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 73dae64bfc9c..9bad7a16dc61 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -947,10 +947,12 @@ static void __tasklet_schedule_common(struct
tasklet_struct *t,
 	 * is locked before adding it to the list.
 	 */
 	if (test_bit(TASKLET_STATE_SCHED, &t->state)) {
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
 		if (test_and_set_bit(TASKLET_STATE_CHAINED, &t->state)) 
{
 			tasklet_unlock(t);
 			return;
 		}
+#endif
 		t->next = NULL;
 		*head->tail = t;
 		head->tail = &(t->next);
@@ -1044,7 +1046,11 @@ static void tasklet_action_common(struct
softirq_action *a,
 again:
 		t->func(t->data);
 
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
 		while (cmpxchg(&t->state, TASKLET_STATEF_RC, 0) !=
TASKLET_STATEF_RC) {
+#else
+		while (!tasklet_tryunlock(t)) {
+#endif
 			/*
 			 * If it got disabled meanwhile, bail out:
 			 */
-- 
2.17.1