Re: [PATCH V3 04/10] sched/core: Prevent race condition between cpuset and __sched_setscheduler()

[Juri Lelli <juri.lelli@redhat.com>]

On 14/02/18 11:49, Juri Lelli wrote:
> On 14/02/18 11:36, Juri Lelli wrote:
> > Hi Mathieu,
> > 
> > On 13/02/18 13:32, Mathieu Poirier wrote:
> > > No synchronisation mechanism exist between the cpuset subsystem and calls
> > > to function __sched_setscheduler().  As such it is possible that new root
> > > domains are created on the cpuset side while a deadline acceptance test
> > > is carried out in __sched_setscheduler(), leading to a potential oversell
> > > of CPU bandwidth.
> > > 
> > > By making available the cpuset_mutex to the core scheduler it is possible
> > > to prevent situations such as the one described above from happening.
> > > 
> > > Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> > > ---
> > 
> > [...]
> > 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index f727c3d0064c..0d8badcf1f0f 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -4176,6 +4176,13 @@ static int __sched_setscheduler(struct task_struct *p,
> > >  	}
> > >  
> > >  	/*
> > > +	 * Make sure we don't race with the cpuset subsystem where root
> > > +	 * domains can be rebuilt or modified while operations like DL
> > > +	 * admission checks are carried out.
> > > +	 */
> > > +	cpuset_lock();
> > > +
> > > +	/*
> > 
> > Mmm, I'm afraid we can't do this. __sched_setscheduler might be called
> > from interrupt contex by normalize_rt_tasks().
> 
> Maybe conditionally grabbing it if pi is true could do? I guess we don't
> care much about domains when sysrq.

Ops.. just got this. :/

--->8---
[    0.020203] ======================================================
[    0.020946] WARNING: possible circular locking dependency detected
[    0.021000] 4.16.0-rc1+ #64 Not tainted
[    0.021000] ------------------------------------------------------
[    0.021000] swapper/0/1 is trying to acquire lock:
[    0.021000]  (cpu_hotplug_lock.rw_sem){.+.+}, at: [<000000007164d41d>] smpboot_register_percpu_thread_cpumask+0x2d/0x100
[    0.021000]
[    0.021000] but task is already holding lock:
[    0.021000]  (cpuset_mutex){+.+.}, at: [<000000008529a52c>] __sched_setscheduler+0xb5/0x8b0
[    0.021000]
[    0.021000] which lock already depends on the new lock.
[    0.021000]
[    0.021000]
[    0.021000] the existing dependency chain (in reverse order) is:
[    0.021000]
[    0.021000] -> #2 (cpuset_mutex){+.+.}:
[    0.021000]        __sched_setscheduler+0xb5/0x8b0
[    0.021000]        _sched_setscheduler+0x6c/0x80
[    0.021000]        __kthread_create_on_node+0x10e/0x170
[    0.021000]        kthread_create_on_node+0x37/0x40
[    0.021000]        kthread_create_on_cpu+0x27/0x90
[    0.021000]        __smpboot_create_thread.part.3+0x64/0xe0
[    0.021000]        smpboot_register_percpu_thread_cpumask+0x91/0x100
[    0.021000]        spawn_ksoftirqd+0x37/0x40
[    0.021000]        do_one_initcall+0x3b/0x160
[    0.021000]        kernel_init_freeable+0x118/0x258
[    0.021000]        kernel_init+0xa/0x100
[    0.021000]        ret_from_fork+0x3a/0x50
[    0.021000]
[    0.021000] -> #1 (smpboot_threads_lock){+.+.}:
[    0.021000]        smpboot_register_percpu_thread_cpumask+0x3b/0x100
[    0.021000]        spawn_ksoftirqd+0x37/0x40
[    0.021000]        do_one_initcall+0x3b/0x160
[    0.021000]        kernel_init_freeable+0x118/0x258
[    0.021000]        kernel_init+0xa/0x100
[    0.021000]        ret_from_fork+0x3a/0x50
[    0.021000]
[    0.021000] -> #0 (cpu_hotplug_lock.rw_sem){.+.+}:
[    0.021000]        cpus_read_lock+0x3e/0x80
[    0.021000]        smpboot_register_percpu_thread_cpumask+0x2d/0x100
[    0.021000]        lockup_detector_init+0x3e/0x74
[    0.021000]        kernel_init_freeable+0x146/0x258
[    0.021000]        kernel_init+0xa/0x100
[    0.021000]        ret_from_fork+0x3a/0x50
[    0.021000]
[    0.021000] other info that might help us debug this:
[    0.021000]
[    0.021000] Chain exists of:
[    0.021000]   cpu_hotplug_lock.rw_sem --> smpboot_threads_lock --> cpuset_mutex
[    0.021000]
[    0.021000]  Possible unsafe locking scenario:
[    0.021000]
[    0.021000]        CPU0                    CPU1
[    0.021000]        ----                    ----
[    0.021000]   lock(cpuset_mutex);
[    0.021000]                                lock(smpboot_threads_lock);
[    0.021000]                                lock(cpuset_mutex);
[    0.021000]   lock(cpu_hotplug_lock.rw_sem);
[    0.021000]
[    0.021000]  *** DEADLOCK ***
[    0.021000]
[    0.021000] 1 lock held by swapper/0/1:
[    0.021000]  #0:  (cpuset_mutex){+.+.}, at: [<000000008529a52c>] __sched_setscheduler+0xb5/0x8b0
[    0.021000]
[    0.021000] stack backtrace:
[    0.021000] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.16.0-rc1+ #64
[    0.021000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-2.fc27 04/01/2014
[    0.021000] Call Trace:
[    0.021000]  dump_stack+0x85/0xc5
[    0.021000]  print_circular_bug.isra.38+0x1ce/0x1db
[    0.021000]  __lock_acquire+0x1278/0x1320
[    0.021000]  ? sched_clock_local+0x12/0x80
[    0.021000]  ? lock_acquire+0x9f/0x1f0
[    0.021000]  lock_acquire+0x9f/0x1f0
[    0.021000]  ? smpboot_register_percpu_thread_cpumask+0x2d/0x100
[    0.021000]  cpus_read_lock+0x3e/0x80
[    0.021000]  ? smpboot_register_percpu_thread_cpumask+0x2d/0x100
[    0.021000]  smpboot_register_percpu_thread_cpumask+0x2d/0x100
[    0.021000]  ? set_debug_rodata+0x11/0x11
[    0.021000]  lockup_detector_init+0x3e/0x74
[    0.021000]  kernel_init_freeable+0x146/0x258
[    0.021000]  ? rest_init+0xd0/0xd0
[    0.021000]  kernel_init+0xa/0x100
[    0.021000]  ret_from_fork+0x3a/0x50