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

[Wanpeng Li]

On 8/25/15 4:13 PM, Leo Yan wrote:
> Hi Wanpeng,
>
> On Tue, Aug 25, 2015 at 03:59:54PM +0800, Wanpeng Li wrote:
> > [   15.273708] ------------[ cut here ]------------
> > [   15.274097] WARNING: CPU: 0 PID: 13 at kernel/sched/core.c:1156 do_set_cpus_allowed+0x7e/0x80()
> > [   15.274857] Modules linked in:
> > [   15.275101] CPU: 0 PID: 13 Comm: migration/0 Not tainted 4.2.0-rc1-00049-g25834c7 #2
> > [   15.275674]  00000000 00000000 d21f1d24 c19228b2 00000000 d21f1d58 c1056a3b c1ba00e4
> > [   15.276084]  00000000 0000000d c1ba17d8 00000484 c10838be 00000484 c10838be d21e5000
> > [   15.276084]  d2121900 d21e5158 d21f1d68 c1056b12 00000009 00000000 d21f1d7c c10838be
> > [   15.276084] Call Trace:
> > [   15.276084]  [<c19228b2>] dump_stack+0x4b/0x75
> > [   15.276084]  [<c1056a3b>] warn_slowpath_common+0x8b/0xc0
> > [   15.276084]  [<c10838be>] ? do_set_cpus_allowed+0x7e/0x80
> > [   15.276084]  [<c10838be>] ? do_set_cpus_allowed+0x7e/0x80
> > [   15.276084]  [<c1056b12>] warn_slowpath_null+0x22/0x30
> > [   15.276084]  [<c10838be>] do_set_cpus_allowed+0x7e/0x80
> > [   15.276084]  [<c110154c>] cpuset_cpus_allowed_fallback+0x7c/0x170
> > [   15.276084]  [<c11014d0>] ? cpuset_cpus_allowed+0x180/0x180
> > [   15.276084]  [<c1083ae1>] select_fallback_rq+0x221/0x280
> > [   15.276084]  [<c1085073>] migration_call+0xe3/0x250
> > [   15.276084]  [<c1079e23>] notifier_call_chain+0x53/0x70
> > [   15.276084]  [<c1079e5e>] __raw_notifier_call_chain+0x1e/0x30
> > [   15.276084]  [<c1056cc8>] cpu_notify+0x28/0x50
> > [   15.276084]  [<c191e4d2>] take_cpu_down+0x22/0x40
> > [   15.276084]  [<c1102895>] multi_cpu_stop+0xd5/0x140
> > [   15.276084]  [<c11027c0>] ? __stop_cpus+0x80/0x80
> > [   15.276084]  [<c11025cc>] cpu_stopper_thread+0xbc/0x170
> > [   15.276084]  [<c1085ec9>] ? preempt_count_sub+0x9/0x50
> > [   15.276084]  [<c192b6a7>] ? _raw_spin_unlock_irq+0x37/0x50
> > [   15.276084]  [<c192b655>] ? _raw_spin_unlock_irqrestore+0x55/0x70
> > [   15.276084]  [<c10a9074>] ? trace_hardirqs_on_caller+0x144/0x1e0
> > [   15.276084]  [<c11024a5>] ? cpu_stop_should_run+0x35/0x40
> > [   15.276084]  [<c1085ec9>] ? preempt_count_sub+0x9/0x50
> > [   15.276084]  [<c192b641>] ? _raw_spin_unlock_irqrestore+0x41/0x70
> > [   15.276084]  [<c107c944>] smpboot_thread_fn+0x174/0x2f0
> > [   15.276084]  [<c107c7d0>] ? sort_range+0x30/0x30
> > [   15.276084]  [<c1078934>] kthread+0xc4/0xe0
> > [   15.276084]  [<c192c041>] ret_from_kernel_thread+0x21/0x30
> > [   15.276084]  [<c1078870>] ? kthread_create_on_node+0x180/0x180
> > [   15.276084] ---[ end trace 15f4c86d404693b0 ]---
> >
> > After commit (25834c73f93: sched: Fix a race between __kthread_bind()
> > and sched_setaffinity()) do_set_cpus_allowed() should be called w/
> > p->pi_lock held, however, it is not true in cpuset path currently.
> >
> > This patch fix it by holding p->pi_lock in cpuset path.
> >
> > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> > ---
> >   kernel/cpuset.c |    4 ++++
> >   1 files changed, 4 insertions(+), 0 deletions(-)
> >
> > diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> > index e414ae9..605ed66 100644
> > --- a/kernel/cpuset.c
> > +++ b/kernel/cpuset.c
> > @@ -2376,8 +2376,12 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
> >   
> >   void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
> >   {
> > +	unsigned long flags;
> > +
> >   	rcu_read_lock();
> > +	raw_spin_lock_irqsave(&tsk->pi_lock, flags);
> >   	do_set_cpus_allowed(tsk, task_cs(tsk)->effective_cpus);
> > +	raw_spin_lock_irqsave(&tsk->pi_lock, flags);
> Just curious, Will introduce deadlock after acquire lock twice? ;)

Could you point out where acquires lock twice?

Regards,
Wanpeng Li

> Thanks,
> Leo Yan
>
> >   	rcu_read_unlock();
> >   
> >   	/*
> > --
> > 1.7.1
> >
> > --
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