Commit 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
removed any path which could make pick_next_rt_entity() return NULL.
However, BUG_ON(!rt_se) in _pick_next_task_rt() (the only caller of
pick_next_rt_entity()) still checks the error condition, which can
never happen, since list_entry() never returns NULL.
Remove the BUG_ON check, and instead emit a warning in the only
possible error condition here: the queue being empty which should
never happen.

Fixes: 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
---
Changes in v3:
- return NULL if the list is empty
- Link to v2: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v2-1-d8e010cce91b@diag.uniroma1.it

Changes in v2:
- pick_next_rt_entity(): emit warning instead of crashing
- Link to v1: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v1-1-c93085ee0055@diag.uniroma1.it
---
 kernel/sched/rt.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index ed2a47e4ddae..0a11f44adee5 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1777,6 +1777,8 @@ static struct sched_rt_entity *pick_next_rt_entity(struct rt_rq *rt_rq)
 	BUG_ON(idx >= MAX_RT_PRIO);
 
 	queue = array->queue + idx;
+	if (SCHED_WARN_ON(list_empty(queue)))
+		return NULL;
 	next = list_entry(queue->next, struct sched_rt_entity, run_list);
 
 	return next;
@@ -1789,7 +1791,8 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
 
 	do {
 		rt_se = pick_next_rt_entity(rt_rq);
-		BUG_ON(!rt_se);
+		if (unlikely(!rt_se))
+			return NULL;
 		rt_rq = group_rt_rq(rt_se);
 	} while (rt_rq);
 

---
base-commit: 2241ab53cbb5cdb08a6b2d4688feb13971058f65
change-id: 20230128-list-entry-null-check-sched-a3f3dfd6d468

Best regards,
-- 
Pietro Borrello <borrello@diag.uniroma1.it>

On Mon, 06 Feb 2023 22:33:54 +0000
Pietro Borrello <borrello@diag.uniroma1.it> wrote:

> Commit 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
> removed any path which could make pick_next_rt_entity() return NULL.
> However, BUG_ON(!rt_se) in _pick_next_task_rt() (the only caller of
> pick_next_rt_entity()) still checks the error condition, which can
> never happen, since list_entry() never returns NULL.
> Remove the BUG_ON check, and instead emit a warning in the only
> possible error condition here: the queue being empty which should
> never happen.
> 
> Fixes: 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
> Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
> ---
> Changes in v3:
> - return NULL if the list is empty
> - Link to v2: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v2-1-d8e010cce91b@diag.uniroma1.it
> 
> Changes in v2:
> - pick_next_rt_entity(): emit warning instead of crashing
> - Link to v1: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v1-1-c93085ee0055@diag.uniroma1.it
> ---
>  kernel/sched/rt.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ed2a47e4ddae..0a11f44adee5 100644

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve

Hi Pietro,

On Mon, Feb 06, 2023 at 10:33:54PM +0000 Pietro Borrello wrote:
> Commit 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
> removed any path which could make pick_next_rt_entity() return NULL.
> However, BUG_ON(!rt_se) in _pick_next_task_rt() (the only caller of
> pick_next_rt_entity()) still checks the error condition, which can
> never happen, since list_entry() never returns NULL.
> Remove the BUG_ON check, and instead emit a warning in the only
> possible error condition here: the queue being empty which should
> never happen.
> 
> Fixes: 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
> Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
> ---
> Changes in v3:
> - return NULL if the list is empty
> - Link to v2: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v2-1-d8e010cce91b@diag.uniroma1.it
> 
> Changes in v2:
> - pick_next_rt_entity(): emit warning instead of crashing
> - Link to v1: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v1-1-c93085ee0055@diag.uniroma1.it
> ---
>  kernel/sched/rt.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ed2a47e4ddae..0a11f44adee5 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1777,6 +1777,8 @@ static struct sched_rt_entity *pick_next_rt_entity(struct rt_rq *rt_rq)
>  	BUG_ON(idx >= MAX_RT_PRIO);
>  
>  	queue = array->queue + idx;
> +	if (SCHED_WARN_ON(list_empty(queue)))
> +		return NULL;
>  	next = list_entry(queue->next, struct sched_rt_entity, run_list);
>  
>  	return next;
> @@ -1789,7 +1791,8 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
>  
>  	do {
>  		rt_se = pick_next_rt_entity(rt_rq);
> -		BUG_ON(!rt_se);
> +		if (unlikely(!rt_se))
> +			return NULL;
>  		rt_rq = group_rt_rq(rt_se);
>  	} while (rt_rq);
>  
> 
> ---
> base-commit: 2241ab53cbb5cdb08a6b2d4688feb13971058f65
> change-id: 20230128-list-entry-null-check-sched-a3f3dfd6d468
> 
> Best regards,
> -- 
> Pietro Borrello <borrello@diag.uniroma1.it>
> 


Reviewed-by: Phil Auld <pauld@redhat.com>

Thanks. I think this will be useful.


Cheers,
Phil


--

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     7c4a5b89a0b5a57a64b601775b296abf77a9fe97
Gitweb:        https://git.kernel.org/tip/7c4a5b89a0b5a57a64b601775b296abf77a9fe97
Author:        Pietro Borrello <borrello@diag.uniroma1.it>
AuthorDate:    Mon, 06 Feb 2023 22:33:54 
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Sat, 11 Feb 2023 11:18:10 +01:00

sched/rt: pick_next_rt_entity(): check list_entry

Commit 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
removed any path which could make pick_next_rt_entity() return NULL.
However, BUG_ON(!rt_se) in _pick_next_task_rt() (the only caller of
pick_next_rt_entity()) still checks the error condition, which can
never happen, since list_entry() never returns NULL.
Remove the BUG_ON check, and instead emit a warning in the only
possible error condition here: the queue being empty which should
never happen.

Fixes: 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Phil Auld <pauld@redhat.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v3-1-b1a71bd1ac6b@diag.uniroma1.it
---
 kernel/sched/rt.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index ed2a47e..0a11f44 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1777,6 +1777,8 @@ static struct sched_rt_entity *pick_next_rt_entity(struct rt_rq *rt_rq)
 	BUG_ON(idx >= MAX_RT_PRIO);
 
 	queue = array->queue + idx;
+	if (SCHED_WARN_ON(list_empty(queue)))
+		return NULL;
 	next = list_entry(queue->next, struct sched_rt_entity, run_list);
 
 	return next;
@@ -1789,7 +1791,8 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
 
 	do {
 		rt_se = pick_next_rt_entity(rt_rq);
-		BUG_ON(!rt_se);
+		if (unlikely(!rt_se))
+			return NULL;
 		rt_rq = group_rt_rq(rt_se);
 	} while (rt_rq);