linux-kernel - [PATCH 2/2] RT: remove "paranoid" limit in push_rt

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Message-ID: <20081003124305.17387.90233.stgit@dev.haskins.net>
Date:	Fri, 03 Oct 2008 08:43:06 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Chirag Jog <chirag@...ux.vnet.ibm.com>
Cc:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, dvhltc@...ibm.com, dino@...ibm.com,
	Gilles.Carry@...l.net
Subject: [PATCH 2/2] RT: remove "paranoid" limit in push_rt_task

A panic was discovered by Chirag Jog and investigated by Gilles Carry
to be originating in the fact that a task being pushed away
may get migrated away during a double_lock_balance.  The result was
that the pushable_tasks list may become corrupted.

The root cause is that the "paranoid" retry limit could cause us to
bail out of a retry, but still try to remove the item from the (now
potentially incorrect) list.  There are numerous ways to correct the
condition, but the paranoid feature is no longer relevant with the new
pushable logic (since pushable naturally limits the loop anyway), so
lets just remove it.

Reported By: Chirag Jog <chirag@...ux.vnet.ibm.com>
Found-by: Gilles Carry <gilles.carry@...l.net>
Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
---

 kernel/sched_rt.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 59ead84..5a754fe 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1056,7 +1056,6 @@ static int push_rt_task(struct rq *rq)
 {
 	struct task_struct *next_task;
 	struct rq *lowest_rq;
-	int paranoid = RT_MAX_TRIES;
 
 	if (!rq->rt.overloaded)
 		return 0;
@@ -1094,12 +1093,14 @@ static int push_rt_task(struct rq *rq)
 		 * If it has, then try again.
 		 */
 		task = pick_next_pushable_task(rq);
-		if (unlikely(task != next_task) && task && paranoid--) {
+		if (unlikely(task != next_task) && task) {
 			put_task_struct(next_task);
 			next_task = task;
 			goto retry;
 		}
 
+		BUG_ON(task_cpu(next_task) != rq->cpu);
+
 		/*
 		 * Once we have failed to push this task, we will not
 		 * try again, since the other cpus will pull from us

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