linux-kernel - [PATCH -rt] Don't let raise_softirq_prio lower the prio (was: [RT] rt priority losing)

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Date:	Mon, 24 Jul 2006 13:15:01 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Esben Nielsen <nielsen.esben@...glemail.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"Duetsch, Thomas LDE1" <thomas.duetsch@...mens.com>
Subject: [PATCH -rt] Don't let raise_softirq_prio lower the prio (was: [RT]
	rt priority losing)

On Mon, 2006-07-24 at 18:54 +0100, Esben Nielsen wrote:

> >
> > OK, you are right about this.  The PI chain should not be affected.  But
> > this could still be a problem if the softirq was running at a high prio
> > for a task when a lower prio callback needs to be made.  It looks like
> > timer is removed from the base before the function runs.  So when the
> > interrupt looks at the base to determine the priority to set it at, it
> > might actually lower the priority of a running hrtimer thread.
> >
> 
> That is a simple bug which ought to be simple fixable.

I guess the simple fix is not to allow the interrupt to lower the
priority. But simple fixes do not always handle all the cases.

Thomas G., see any side effects with this patch?

-- Steve

Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Index: linux-2.6.17-rt7/kernel/softirq.c
===================================================================
--- linux-2.6.17-rt7.orig/kernel/softirq.c	2006-07-24 13:08:53.000000000 -0400
+++ linux-2.6.17-rt7/kernel/softirq.c	2006-07-24 13:10:13.000000000 -0400
@@ -108,7 +108,12 @@ static void wakeup_softirqd_prio(int sof
 	struct task_struct *tsk = __get_cpu_var(ksoftirqd[softirq].tsk);
 
 	if (tsk) {
-		if (tsk->normal_prio != prio) {
+		/*
+		 * The lower the prio, the higher the priority.
+		 * This can only raise the priority but it can
+		 * not lower it.
+		 */
+		if (tsk->normal_prio > prio) {
 			struct sched_param param;
 
 			param.sched_priority = MAX_RT_PRIO-1 - prio;


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