Re: [PATCH v2] sched: fix tsk->pi_lock isn't held when do_set_cpus_allowed()

From: Wanpeng Li
Date: Fri Aug 28 2015 - 02:43:40 EST


Hi Boqun,
On 8/28/15 2:33 PM, Boqun Feng wrote:
Hi Wanpeng,

On Fri, Aug 28, 2015 at 12:02:47PM +0800, Wanpeng Li wrote:
<snip>
This patch fix it by following the rules for changing task_struct::cpus_allowed
w/ both pi_lock and rq->lock are held.

Reported-by: kernel test robot <ying.huang@xxxxxxxxx>
Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
v1 -> v2:
* fix the silly double lock stuff
* follow the rules for changing task_struct::cpus_allowed

kernel/sched/core.c | 22 ++++++++++++++++++++++
1 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b3386c6..8cf87e3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5186,6 +5186,27 @@ static void migrate_tasks(struct rq *dead_rq)
BUG_ON(!next);
next->sched_class->put_prev_task(rq, next);
+ /*
+ * Rules for changing task_struct::cpus_allowed are holding
+ * both pi_lock and rq->lock, such that holding either
+ * stabilizes the mask.
+ *
+ * Drop rq->lock is not quite as disastrous as it usually is
+ * because !cpu_active at this point, which means load-balance
+ * will not interfere.
+ */
+ lockdep_unpin_lock(&rq->lock);
+ raw_spin_unlock(&rq->lock);
+ raw_spin_lock(&next->pi_lock);
+ raw_spin_lock(&rq->lock);
+ lockdep_pin_lock(&rq->lock);
+ if (!(task_rq(next) == rq && task_on_rq_queued(next))) {
+ lockdep_unpin_lock(&rq->lock);
+ raw_spin_unlock(&rq->lock);
Dropping rq->lock here means we will continue the loop without the
rq->lock, right? But we do have a lockdep_pin_lock(&rq->lock) in the
beginning of every iteration. So can we release rq->lock here?

Good catch! There is no need to lockdep_unpin and unlock rq->lock I think.

Regards,
Wanpeng Li


Regards,
Boqun

+ raw_spin_unlock(&next->pi_lock);
+ continue;
+ }
+
/* Find suitable destination for @next, with force if needed. */
dest_cpu = select_fallback_rq(dead_rq->cpu, next);
@@ -5196,6 +5217,7 @@ static void migrate_tasks(struct rq *dead_rq)
rq = dead_rq;
raw_spin_lock(&rq->lock);
}
+ raw_spin_unlock(&next->pi_lock);
}
rq->stop = stop;
--
1.7.1

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