2018-11-03 18:34:25

by Olof Johansson

Subject: [PATCH] psi: simplify cgroup_move_task

The existing code triggered an invalid warning about 'rq' possibly being
used uninitialized. Instead of doing the silly warning suppression by
initializa it to NULL, refactor the code to bail out early instead.

Warning was:

kernel/sched/psi.c: In function ‘cgroup_move_task’:
kernel/sched/psi.c:639:13: warning: ‘rq’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Fixes: 2ce7135adc9ad ("psi: cgroup support")
Cc: Johannes Weiner <[email protected]>
Signed-off-by: Olof Johansson <[email protected]>
---
kernel/sched/psi.c | 43 ++++++++++++++++++++++---------------------
1 file changed, 22 insertions(+), 21 deletions(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 7cdecfc010af..3d7355d7c3e3 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -633,38 +633,39 @@ void psi_cgroup_free(struct cgroup *cgroup)
*/
void cgroup_move_task(struct task_struct *task, struct css_set *to)
{
- bool move_psi = !psi_disabled;
unsigned int task_flags = 0;
struct rq_flags rf;
struct rq *rq;

- if (move_psi) {
- rq = task_rq_lock(task, &rf);
+ if (psi_disabled) {
+ /*
+ * Lame to do this here, but the scheduler cannot be locked
+ * from the outside, so we move cgroups from inside sched/.
+ */
+ rcu_assign_pointer(task->cgroups, to);
+ return;
+ }

- if (task_on_rq_queued(task))
- task_flags = TSK_RUNNING;
- else if (task->in_iowait)
- task_flags = TSK_IOWAIT;
+ rq = task_rq_lock(task, &rf);

- if (task->flags & PF_MEMSTALL)
- task_flags |= TSK_MEMSTALL;
+ if (task_on_rq_queued(task))
+ task_flags = TSK_RUNNING;
+ else if (task->in_iowait)
+ task_flags = TSK_IOWAIT;

- if (task_flags)
- psi_task_change(task, task_flags, 0);
- }
+ if (task->flags & PF_MEMSTALL)
+ task_flags |= TSK_MEMSTALL;

- /*
- * Lame to do this here, but the scheduler cannot be locked
- * from the outside, so we move cgroups from inside sched/.
- */
+ if (task_flags)
+ psi_task_change(task, task_flags, 0);
+
+ /* See comment above */
rcu_assign_pointer(task->cgroups, to);

- if (move_psi) {
- if (task_flags)
- psi_task_change(task, 0, task_flags);
+ if (task_flags)
+ psi_task_change(task, 0, task_flags);

- task_rq_unlock(rq, task, &rf);
- }
+ task_rq_unlock(rq, task, &rf);
}
#endif /* CONFIG_CGROUPS */

--
2.11.0

2018-11-06 00:31:47

by Andrew Morton

Subject: Re: [PATCH] psi: simplify cgroup_move_task

On Sat, 3 Nov 2018 11:33:39 -0700 Olof Johansson <[email protected]> wrote:

> The existing code triggered an invalid warning about 'rq' possibly being
> used uninitialized. Instead of doing the silly warning suppression by
> initializa it to NULL, refactor the code to bail out early instead.
>
> Warning was:
>
> kernel/sched/psi.c: In function ‘cgroup_move_task’:
> kernel/sched/psi.c:639:13: warning: ‘rq’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -633,38 +633,39 @@ void psi_cgroup_free(struct cgroup *cgroup)
> */
> void cgroup_move_task(struct task_struct *task, struct css_set *to)
> {
> - bool move_psi = !psi_disabled;
> unsigned int task_flags = 0;
> struct rq_flags rf;
> struct rq *rq;
>
> - if (move_psi) {
> - rq = task_rq_lock(task, &rf);
> + if (psi_disabled) {
> + /*
> + * Lame to do this here, but the scheduler cannot be locked
> + * from the outside, so we move cgroups from inside sched/.
> + */
> + rcu_assign_pointer(task->cgroups, to);
> + return;
> + }

Fair enough.

Surprisingly this doesn't increase psi.o text size.

2018-11-06 16:56:19

by Johannes Weiner

Subject: Re: [PATCH] psi: simplify cgroup_move_task

On Sat, Nov 03, 2018 at 11:33:39AM -0700, Olof Johansson wrote:
> The existing code triggered an invalid warning about 'rq' possibly being
> used uninitialized. Instead of doing the silly warning suppression by
> initializa it to NULL, refactor the code to bail out early instead.
>
> Warning was:
>
> kernel/sched/psi.c: In function ‘cgroup_move_task’:
> kernel/sched/psi.c:639:13: warning: ‘rq’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>
> Fixes: 2ce7135adc9ad ("psi: cgroup support")
> Cc: Johannes Weiner <[email protected]>
> Signed-off-by: Olof Johansson <[email protected]>

Acked-by: Johannes Weiner <[email protected]>

Thanks Olof!