[4.19 stable-rt PATCH] tasklet: Fix UP case for tasklet CHAINED state

[4.19 stable-rt PATCH] tasklet: Fix UP case for tasklet CHAINED state

[Tom Zanussi]

4.19 stable-rt 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 because the RUN flag
is SMP-only.

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

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

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 73dae64bfc9c..4f37a6173ab9 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)
 		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)
+		while (!tasklet_tryunlock(t)) {
+#else
 		while (cmpxchg(&t->state, TASKLET_STATEF_RC, 0) != TASKLET_STATEF_RC) {
+#endif
 			/*
 			 * If it got disabled meanwhile, bail out:
 			 */
-- 
2.17.1

Re: [4.19 stable-rt PATCH] tasklet: Fix UP case for tasklet CHAINED state

[Sebastian Andrzej Siewior]

On 2020-06-09 11:21:44 [-0500], Tom Zanussi wrote:
> index 73dae64bfc9c..4f37a6173ab9 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)
>  		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)
> +		while (!tasklet_tryunlock(t)) {
> +#else
>  		while (cmpxchg(&t->state, TASKLET_STATEF_RC, 0) != TASKLET_STATEF_RC) {
> +#endif

This is still needed for RT && !SMP

>  			/*
>  			 * If it got disabled meanwhile, bail out:
>  			 */

Sebastian

Re: [4.19 stable-rt PATCH] tasklet: Fix UP case for tasklet CHAINED state

[Tom Zanussi]

Hi Sebastian,

On Tue, 2020-06-09 at 18:38 +0200, Sebastian Andrzej Siewior wrote:
> On 2020-06-09 11:21:44 [-0500], Tom Zanussi wrote:
> > index 73dae64bfc9c..4f37a6173ab9 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)
> >  		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)
> > +		while (!tasklet_tryunlock(t)) {
> > +#else
> >  		while (cmpxchg(&t->state, TASKLET_STATEF_RC, 0) !=
> > TASKLET_STATEF_RC) {
> > +#endif
> 
> This is still needed for RT && !SMP
> 

Yes, you're right - I'll change this and repost after testing all
cases.

Thanks,

Tom

> >  			/*
> >  			 * If it got disabled meanwhile, bail out:
> >  			 */
> 
> Sebastian