Re: [PATCH v3] sched: fix tsk->pi_lock isn't held when do_set_cpus_allowed()
From: Wanpeng Li
Date: Fri Aug 28 2015 - 09:58:34 EST
On 8/28/15 9:29 PM, Peter Zijlstra wrote:
On Fri, Aug 28, 2015 at 02:55:56PM +0800, Wanpeng Li wrote:
This patch fix it by following the rules for changing task_struct::cpus_allowed
w/ both pi_lock and rq->lock are held.
Thanks, I made that the below. There was a pin leak and I turned the
safety check into a WARN_ON because it really should not happen.
I also munged some of the comments a bit and did some slight edits to
the Changelog.
Cool, thanks for the help. :-)
Regards,
Wanpeng Li
---
Subject: sched: 'Annotate' migrate_tasks()
From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
Date: Fri, 28 Aug 2015 14:55:56 +0800
| WARNING: CPU: 0 PID: 13 at kernel/sched/core.c:1156 do_set_cpus_allowed+0x7e/0x80()
| Modules linked in:
| CPU: 0 PID: 13 Comm: migration/0 Not tainted 4.2.0-rc1-00049-g25834c7 #2
| Call Trace:
| dump_stack+0x4b/0x75
| warn_slowpath_common+0x8b/0xc0
| warn_slowpath_null+0x22/0x30
| do_set_cpus_allowed+0x7e/0x80
| cpuset_cpus_allowed_fallback+0x7c/0x170
| select_fallback_rq+0x221/0x280
| migration_call+0xe3/0x250
| notifier_call_chain+0x53/0x70
| __raw_notifier_call_chain+0x1e/0x30
| cpu_notify+0x28/0x50
| take_cpu_down+0x22/0x40
| multi_cpu_stop+0xd5/0x140
| cpu_stopper_thread+0xbc/0x170
| smpboot_thread_fn+0x174/0x2f0
| kthread+0xc4/0xe0
| ret_from_kernel_thread+0x21/0x30
As Peterz pointed out:
| So the normal rules for changing task_struct::cpus_allowed are holding
| both pi_lock and rq->lock, such that holding either stabilizes the mask.
|
| This is so that wakeup can happen without rq->lock and load-balance
| without pi_lock.
|
| From this we already get the relaxation that we can omit acquiring
| rq->lock if the task is not on the rq, because in that case
| load-balancing will not apply to it.
|
| ** these are the rules currently tested in do_set_cpus_allowed() **
|
| Now, since __set_cpus_allowed_ptr() uses task_rq_lock() which
| unconditionally acquires both locks, we could get away with holding just
| rq->lock when on_rq for modification because that'd still exclude
| __set_cpus_allowed_ptr(), it would also work against
| __kthread_bind_mask() because that assumes !on_rq.
|
| That said, this is all somewhat fragile.
|
| Now, I don't think dropping rq->lock is quite as disastrous as it
| usually is because !cpu_active at this point, which means load-balance
| will not interfere, but that too is somewhat fragile.
|
| So we end up with a choice of two fragile..
This patch fixes it by following the rules for changing
task_struct::cpus_allowed with both pi_lock and rq->lock held.
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Reported-by: kernel test robot <ying.huang@xxxxxxxxx>
Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
[Modified changelog and patch]
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/BLU436-SMTP1660820490DE202E3934ED3806E0@xxxxxxx
---
kernel/sched/core.c | 29 ++++++++++++++++++++++++++---
1 file changed, 26 insertions(+), 3 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5178,24 +5178,47 @@ static void migrate_tasks(struct rq *dea
break;
/*
- * Ensure rq->lock covers the entire task selection
- * until the migration.
+ * pick_next_task assumes pinned rq->lock.
*/
lockdep_pin_lock(&rq->lock);
next = pick_next_task(rq, &fake_task);
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. Also, stop-machine.
+ */
+ lockdep_unpin_lock(&rq->lock);
+ raw_spin_unlock(&rq->lock);
+ raw_spin_lock(&next->pi_lock);
+ raw_spin_lock(&rq->lock);
+
+ /*
+ * Since we're inside stop-machine, _nothing_ should have
+ * changed the task, WARN if weird stuff happened, because in
+ * that case the above rq->lock drop is a fail too.
+ */
+ if (WARN_ON(task_rq(next) != rq || !task_on_rq_queued(next))) {
+ 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);
- lockdep_unpin_lock(&rq->lock);
rq = __migrate_task(rq, next, dest_cpu);
if (rq != dead_rq) {
raw_spin_unlock(&rq->lock);
rq = dead_rq;
raw_spin_lock(&rq->lock);
}
+ raw_spin_unlock(&next->pi_lock);
}
rq->stop = stop;
--
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