[PATCH] psi: simplify cgroup_move_task

[PATCH] psi: simplify cgroup_move_task

[Olof Johansson]

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 <hannes@cmpxchg.org>
Signed-off-by: Olof Johansson <olof@lixom.net>
---
 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

Re: [PATCH] psi: simplify cgroup_move_task

[Andrew Morton]

On Sat,  3 Nov 2018 11:33:39 -0700 Olof Johansson <olof@lixom.net> 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.

Re: [PATCH] psi: simplify cgroup_move_task

[Johannes Weiner]

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 <hannes@cmpxchg.org>
> Signed-off-by: Olof Johansson <olof@lixom.net>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks Olof!