Re: Query regarding synchronize_sched_expedited and resched_cpu

From: Paul E. McKenney
Date: Mon Sep 18 2017 - 19:53:20 EST

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@xxxxxxxxxxxxxxxxxx> 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