[PATCH] sched/cpupri: fix cpupri_find() for high priority tasks

[PATCH] sched/cpupri: fix cpupri_find() for high priority tasks

[Mike Galbraith]

Hi Steven,

Seems c92211d9b7727 introduced a buglet.

--snip--

Bail on task_pri >= MAX_RT_PRIO excludes userspace prio 98 and 99 tasks,
which map to 100 and 101 respectively.

A user reported that given two SCHED_RR tasks, one hog, one light, the light
task may be stacked on top of the hog iff prio >= 98, latency hit follows.

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: <stable@vger.kernel.org>
Fixes: c92211d9b7727 sched/cpupri: Remove the vec->lock
---
 kernel/sched/cpupri.c |    3 ---
 1 file changed, 3 deletions(-)

--- a/kernel/sched/cpupri.c
+++ b/kernel/sched/cpupri.c
@@ -70,9 +70,6 @@ int cpupri_find(struct cpupri *cp, struc
 	int idx = 0;
 	int task_pri = convert_prio(p->prio);
 
-	if (task_pri >= MAX_RT_PRIO)
-		return 0;
-
 	for (idx = 0; idx < task_pri; idx++) {
 		struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
 		int skip = 0;

Re: [PATCH] sched/cpupri: fix cpupri_find() for high priority tasks

[Steven Rostedt]

On Wed, 09 Apr 2014 05:50:10 +0200
Mike Galbraith <umgwanakikbuti@gmail.com> wrote:

> Hi Steven,
> 
> Seems c92211d9b7727 introduced a buglet.
> 
> --snip--
> 
> Bail on task_pri >= MAX_RT_PRIO excludes userspace prio 98 and 99 tasks,
> which map to 100 and 101 respectively.
> 
> A user reported that given two SCHED_RR tasks, one hog, one light, the light
> task may be stacked on top of the hog iff prio >= 98, latency hit follows.
> 
> Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
> Cc: <stable@vger.kernel.org>
> Fixes: c92211d9b7727 sched/cpupri: Remove the vec->lock
> ---
>  kernel/sched/cpupri.c |    3 ---
>  1 file changed, 3 deletions(-)
> 
> --- a/kernel/sched/cpupri.c
> +++ b/kernel/sched/cpupri.c
> @@ -70,9 +70,6 @@ int cpupri_find(struct cpupri *cp, struc
>  	int idx = 0;
>  	int task_pri = convert_prio(p->prio);
>  
> -	if (task_pri >= MAX_RT_PRIO)
> -		return 0;

task_pri is used as an index into pri_to_cpu. Although I don't see how
this can be called for a non RT task, the safer solution is below.

-- Steve

> -
>  	for (idx = 0; idx < task_pri; idx++) {
>  		struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
>  		int skip = 0;
> 

>From 606aa467c9a51c09ce4efa320db45eee59a9bd06 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
Date: Sun, 13 Apr 2014 09:34:53 -0400
Subject: [PATCH] sched: Use CPUPRI_NR_PRIORITIES instead of MAX_RT_PRIO in
 cpupri check

The check at the beginning of cpupri_find() makes sure that the task_pri
variable does not exceed the cp->pri_to_cpu array length. But that length
is CPUPRI_NR_PRIORITIES not MAX_RT_PRIO, where it will miss the last two
priorities in that array.

Link: http://lkml.kernel.org/r/1397015410.5212.13.camel@marge.simpson.net

Cc: stable@vger.kernel.org
Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/sched/cpupri.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
index 8b836b3..557356a 100644
--- a/kernel/sched/cpupri.c
+++ b/kernel/sched/cpupri.c
@@ -70,7 +70,7 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
 	int idx = 0;
 	int task_pri = convert_prio(p->prio);
 
-	if (task_pri >= MAX_RT_PRIO)
+	if (task_pri >= CPUPRI_NR_PRIORITIES)
 		return 0;
 
 	for (idx = 0; idx < task_pri; idx++) {
-- 
1.8.1.4

Re: [PATCH] sched/cpupri: fix cpupri_find() for high priority tasks

[Steven Rostedt]

Actually, since task_pri is calculated from convert_prio() that check
should never be hit. But, I'm paranoid, and instead of dropping the
check, we can do the same as what cpupri_set() does. Which is to BUG.

-- Steve

>From aac271901d6ef5ad19c52f166bf130ad27515b5f Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
Date: Sun, 13 Apr 2014 09:34:53 -0400
Subject: [PATCH] sched: Use CPUPRI_NR_PRIORITIES instead of MAX_RT_PRIO in
 cpupri check

The check at the beginning of cpupri_find() makes sure that the task_pri
variable does not exceed the cp->pri_to_cpu array length. But that length
is CPUPRI_NR_PRIORITIES not MAX_RT_PRIO, where it will miss the last two
priorities in that array.

As task_pri is computed from convert_prio() which should never be bigger
than CPUPRI_NR_PRIORITIES, if the check should cause a panic if it is
hit.

Link: http://lkml.kernel.org/r/1397015410.5212.13.camel@marge.simpson.net

Cc: stable@vger.kernel.org
Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/sched/cpupri.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
index 8b836b3..3031bac 100644
--- a/kernel/sched/cpupri.c
+++ b/kernel/sched/cpupri.c
@@ -70,8 +70,7 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
 	int idx = 0;
 	int task_pri = convert_prio(p->prio);
 
-	if (task_pri >= MAX_RT_PRIO)
-		return 0;
+	BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
 
 	for (idx = 0; idx < task_pri; idx++) {
 		struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
-- 
1.8.1.4

[tip:sched/core] sched: Use CPUPRI_NR_PRIORITIES instead of MAX_RT_PRIO in cpupri check

[tip-bot for Steven Rostedt (Red Hat)]

Commit-ID:  6227cb00cc120f9a43ce8313bb0475ddabcb7d01
Gitweb:     http://git.kernel.org/tip/6227cb00cc120f9a43ce8313bb0475ddabcb7d01
Author:     Steven Rostedt (Red Hat) <rostedt@goodmis.org>
AuthorDate: Sun, 13 Apr 2014 09:34:53 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 7 May 2014 11:51:33 +0200

sched: Use CPUPRI_NR_PRIORITIES instead of MAX_RT_PRIO in cpupri check

The check at the beginning of cpupri_find() makes sure that the task_pri
variable does not exceed the cp->pri_to_cpu array length. But that length
is CPUPRI_NR_PRIORITIES not MAX_RT_PRIO, where it will miss the last two
priorities in that array.

As task_pri is computed from convert_prio() which should never be bigger
than CPUPRI_NR_PRIORITIES, if the check should cause a panic if it is
hit.

Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1397015410.5212.13.camel@marge.simpson.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/cpupri.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
index 8b836b3..3031bac 100644
--- a/kernel/sched/cpupri.c
+++ b/kernel/sched/cpupri.c
@@ -70,8 +70,7 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
 	int idx = 0;
 	int task_pri = convert_prio(p->prio);
 
-	if (task_pri >= MAX_RT_PRIO)
-		return 0;
+	BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
 
 	for (idx = 0; idx < task_pri; idx++) {
 		struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];