Re: Query regarding synchronize_sched_expedited and resched_cpu

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Sep 18, 2017 at 09:55:27AM -0700, Paul E. McKenney wrote:
> On Mon, Sep 18, 2017 at 12:29:31PM -0400, Steven Rostedt wrote:
> > On Mon, 18 Sep 2017 09:24:12 -0700
> > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > 
> > 
> > > As soon as I work through the backlog of lockdep complaints that
> > > appeared in the last merge window...  :-(
> > > 
> > > sparse_irq_lock, I am looking at you!!!  ;-)
> > 
> > I just hit one too, and decided to write a patch to show a chain of 3
> > when applicable.
> > 
> > For example:
> > 
> >  Chain exists of:
> >    cpu_hotplug_lock.rw_sem --> smpboot_threads_lock --> (complete)&self->parked
> > 
> >   Possible unsafe locking scenario by crosslock:
> > 
> >         CPU0                    CPU1                    CPU2
> >         ----                    ----                    ----
> >    lock(smpboot_threads_lock);
> >    lock((complete)&self->parked);
> >                                 lock(cpu_hotplug_lock.rw_sem);
> >                                 lock(smpboot_threads_lock);
> >                                                        lock(cpu_hotplug_lock.rw_sem);
> >                                                        unlock((complete)&self->parked);
> > 
> >   *** DEADLOCK ***
> > 
> > :-)
> 
> Nice!!!
> 
> My next step is reverting 12ac1d0f6c3e ("genirq: Make sparse_irq_lock
> protect what it should protect") to see if that helps.

No joy, but it is amazing how much nicer "git bisect" is when your
failure happens deterministically within 35 seconds.  ;-)

The bisection converged to the range starting with 7a46ec0e2f48
("locking/refcounts, x86/asm: Implement fast refcount overflow
protection") and ending with 0c2364791343 ("Merge branch 'x86/asm'
into locking/core").  All of these failed with an unrelated build
error, but there was a fix that could be merged.  This flagged
d0541b0fa64b ("locking/lockdep: Make CONFIG_LOCKDEP_CROSSRELEASE part
of CONFIG_PROVE_LOCKING"), which unfortunately does not revert cleanly.
However, the effect of a reversion can be obtained by removing the
selects of LOCKDEP_CROSSRELEASE and LOCKDEP_COMPLETE from
PROVE_LOCKING, which allows recent commits to complete a short
rcutorture test successfully.

So, Byungchul, any enlightenment?  Please see lockdep splat below.

							Thanx, Paul

------------------------------------------------------------------------

[   35.310179] ======================================================
[   35.310749] WARNING: possible circular locking dependency detected
[   35.310749] 4.13.0-rc4+ #1 Not tainted
[   35.310749] ------------------------------------------------------
[   35.310749] torture_onoff/766 is trying to acquire lock:
[   35.313943]  ((complete)&st->done){+.+.}, at: [<ffffffffb905f5a6>] takedown_cpu+0x86/0xf0
[   35.313943] 
[   35.313943] but task is already holding lock:
[   35.313943]  (sparse_irq_lock){+.+.}, at: [<ffffffffb90c5e42>] irq_lock_sparse+0x12/0x20
[   35.313943] 
[   35.313943] which lock already depends on the new lock.
[   35.313943] 
[   35.313943] 
[   35.313943] the existing dependency chain (in reverse order) is:
[   35.313943] 
[   35.313943] -> #1 (sparse_irq_lock){+.+.}:
[   35.313943]        __mutex_lock+0x65/0x960
[   35.313943]        mutex_lock_nested+0x16/0x20
[   35.313943]        irq_lock_sparse+0x12/0x20
[   35.313943]        irq_affinity_online_cpu+0x13/0xd0
[   35.313943]        cpuhp_invoke_callback+0xa7/0x8b0
[   35.313943] 
[   35.313943] -> #0 ((complete)&st->done){+.+.}:
[   35.313943]        check_prev_add+0x401/0x800
[   35.313943]        __lock_acquire+0x1100/0x11a0
[   35.313943]        lock_acquire+0x9e/0x1e0
[   35.313943]        wait_for_completion+0x36/0x130
[   35.313943]        takedown_cpu+0x86/0xf0
[   35.313943]        cpuhp_invoke_callback+0xa7/0x8b0
[   35.313943]        cpuhp_down_callbacks+0x3d/0x80
[   35.313943]        _cpu_down+0xbb/0xf0
[   35.313943]        do_cpu_down+0x39/0x50
[   35.313943]        cpu_down+0xb/0x10
[   35.313943]        torture_offline+0x75/0x140
[   35.313943]        torture_onoff+0x102/0x1e0
[   35.313943]        kthread+0x142/0x180
[   35.313943]        ret_from_fork+0x27/0x40
[   35.313943] 
[   35.313943] other info that might help us debug this:
[   35.313943] 
[   35.313943]  Possible unsafe locking scenario:
[   35.313943] 
[   35.313943]        CPU0                    CPU1
[   35.313943]        ----                    ----
[   35.313943]   lock(sparse_irq_lock);
[   35.313943]                                lock((complete)&st->done);
[   35.313943]                                lock(sparse_irq_lock);
[   35.313943]   lock((complete)&st->done);
[   35.313943] 
[   35.313943]  *** DEADLOCK ***
[   35.313943] 
[   35.313943] 3 locks held by torture_onoff/766:
[   35.313943]  #0:  (cpu_add_remove_lock){+.+.}, at: [<ffffffffb9060be2>] do_cpu_down+0x22/0x50
[   35.313943]  #1:  (cpu_hotplug_lock.rw_sem){++++}, at: [<ffffffffb90acc41>] percpu_down_write+0x21/0xf0
[   35.313943]  #2:  (sparse_irq_lock){+.+.}, at: [<ffffffffb90c5e42>] irq_lock_sparse+0x12/0x20
[   35.313943] 
[   35.313943] stack backtrace:
[   35.313943] CPU: 7 PID: 766 Comm: torture_onoff Not tainted 4.13.0-rc4+ #1
[   35.313943] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[   35.313943] Call Trace:
[   35.313943]  dump_stack+0x67/0x97
[   35.313943]  print_circular_bug+0x21d/0x330
[   35.313943]  ? add_lock_to_list.isra.31+0xc0/0xc0
[   35.313943]  check_prev_add+0x401/0x800
[   35.313943]  ? wake_up_q+0x70/0x70
[   35.313943]  __lock_acquire+0x1100/0x11a0
[   35.313943]  ? __lock_acquire+0x1100/0x11a0
[   35.313943]  ? add_lock_to_list.isra.31+0xc0/0xc0
[   35.313943]  lock_acquire+0x9e/0x1e0
[   35.313943]  ? takedown_cpu+0x86/0xf0
[   35.313943]  wait_for_completion+0x36/0x130
[   35.313943]  ? takedown_cpu+0x86/0xf0
[   35.313943]  ? stop_machine_cpuslocked+0xb9/0xd0
[   35.313943]  ? cpuhp_invoke_callback+0x8b0/0x8b0
[   35.313943]  ? cpuhp_complete_idle_dead+0x10/0x10
[   35.313943]  takedown_cpu+0x86/0xf0
[   35.313943]  cpuhp_invoke_callback+0xa7/0x8b0
[   35.313943]  cpuhp_down_callbacks+0x3d/0x80
[   35.313943]  _cpu_down+0xbb/0xf0
[   35.313943]  do_cpu_down+0x39/0x50
[   35.313943]  cpu_down+0xb/0x10
[   35.313943]  torture_offline+0x75/0x140
[   35.313943]  torture_onoff+0x102/0x1e0
[   35.313943]  kthread+0x142/0x180
[   35.313943]  ? torture_kthread_stopping+0x70/0x70
[   35.313943]  ? kthread_create_on_node+0x40/0x40
[   35.313943]  ret_from_fork+0x27/0x40