[ moving this to LKML from linux-rt-users, as that's where it should be ]
On Sat, 17 May 2014 05:36:59 +0200
Mike Galbraith <umgwanakikbuti@xxxxxxxxx> wrote:
> 3.14-rt being build with a non-rt config is unlikely, but..
>
> >From 60e69eed85bb7b5198ef70643b5895c26ad76ef7 Mon Sep 17 00:00:00 2001
> From: Mike Galbraith <bitbucket@xxxxxxxxx>
> Date: Mon, 7 Apr 2014 10:55:15 +0200
> Subject: [PATCH] sched/numa: Fix task_numa_free() lockdep splat
>
> Sasha reported that lockdep claims that the following commit:
> made numa_group.lock interrupt unsafe:
>
> 156654f491dd ("sched/numa: Move task_numa_free() to __put_task_struct()")
>
> While I don't see how that could be, given the commit in question moved
> task_numa_free() from one irq enabled region to another, the below does
> make both gripes and lockups upon gripe with numa=fake=4 go away.
It wasn't the irqs that was causing the lockdep splat, but the
softirqs. You moved it into __put_task_struct() which is called as a
rcu callback that gets called from soft irqs. So yes, you need to
prevent softirqs from happening whenever you take the lock.
spin_lock_irq() is a bigger hammer than needed. The patch below should
be good enough.
I kept the double_lock_irq() as there is no double_lock_bh(). Should we
bother to make one?
-- Steve
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7570dd9..f072ea9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1497,7 +1497,7 @@ static void task_numa_placement(struct task_struct *p)
/* If the task is part of a group prevent parallel updates to group stats */
if (p->numa_group) {
group_lock = &p->numa_group->lock;
- spin_lock_irq(group_lock);
+ spin_lock_bh(group_lock);
}
/* Find the node with the highest number of faults */
@@ -1572,7 +1572,7 @@ static void task_numa_placement(struct task_struct *p)
}
}
- spin_unlock_irq(group_lock);
+ spin_unlock_bh(group_lock);
}
/* Preferred node as the node with the most faults */
@@ -1711,14 +1711,14 @@ void task_numa_free(struct task_struct *p)
void *numa_faults = p->numa_faults_memory;
if (grp) {
- spin_lock_irq(&grp->lock);
+ spin_lock_bh(&grp->lock);
for (i = 0; i < NR_NUMA_HINT_FAULT_STATS * nr_node_ids; i++)
grp->faults[i] -= p->numa_faults_memory[i];
grp->total_faults -= p->total_numa_faults;
list_del(&p->numa_entry);
grp->nr_tasks--;
- spin_unlock_irq(&grp->lock);
+ spin_unlock_bh(&grp->lock);
rcu_assign_pointer(p->numa_group, NULL);
put_numa_group(grp);
}
--
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