Re: [PATCH] percpu: remove spurious lock dependency between percpu and sched

From: Eric Dumazet
Date: Wed May 08 2019 - 15:05:30 EST

On Wed, May 8, 2019 at 11:59 AM Dennis Zhou <dennis@xxxxxxxxxx> wrote:
>
> On Tue, May 07, 2019 at 06:43:20PM -0700, John Sperbeck wrote:
> > In free_percpu() we sometimes call pcpu_schedule_balance_work() to
> > queue a work item (which does a wakeup) while holding pcpu_lock.
> > This creates an unnecessary lock dependency between pcpu_lock and
> > the scheduler's pi_lock. There are other places where we call
> > pcpu_schedule_balance_work() without hold pcpu_lock, and this case
> > doesn't need to be different.
> >
> > Moving the call outside the lock prevents the following lockdep splat
> > when running tools/testing/selftests/bpf/{test_maps,test_progs} in
> > sequence with lockdep enabled:
> >
> > ======================================================
> > WARNING: possible circular locking dependency detected
> > 5.1.0-dbg-DEV #1 Not tainted
> > ------------------------------------------------------
> > kworker/23:255/18872 is trying to acquire lock:
> > 000000000bc79290 (&(&pool->lock)->rlock){-.-.}, at: __queue_work+0xb2/0x520
> >
> > but task is already holding lock:
> > 00000000e3e7a6aa (pcpu_lock){..-.}, at: free_percpu+0x36/0x260
> >
> > which lock already depends on the new lock.
> >
> > the existing dependency chain (in reverse order) is:
> >
> > -> #4 (pcpu_lock){..-.}:
> > lock_acquire+0x9e/0x180
> > _raw_spin_lock_irqsave+0x3a/0x50
> > pcpu_alloc+0xfa/0x780
> > __alloc_percpu_gfp+0x12/0x20
> > alloc_htab_elem+0x184/0x2b0
> > __htab_percpu_map_update_elem+0x252/0x290
> > bpf_percpu_hash_update+0x7c/0x130
> > __do_sys_bpf+0x1912/0x1be0
> > __x64_sys_bpf+0x1a/0x20
> > do_syscall_64+0x59/0x400
> > entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >
> > -> #3 (&htab->buckets[i].lock){....}:
> > lock_acquire+0x9e/0x180
> > _raw_spin_lock_irqsave+0x3a/0x50
> > htab_map_update_elem+0x1af/0x3a0
> >
> > -> #2 (&rq->lock){-.-.}:
> > lock_acquire+0x9e/0x180
> > _raw_spin_lock+0x2f/0x40
> > task_fork_fair+0x37/0x160
> > sched_fork+0x211/0x310
> > copy_process.part.43+0x7b1/0x2160
> > _do_fork+0xda/0x6b0
> > kernel_thread+0x29/0x30
> > rest_init+0x22/0x260
> > arch_call_rest_init+0xe/0x10
> > start_kernel+0x4fd/0x520
> > x86_64_start_reservations+0x24/0x26
> > x86_64_start_kernel+0x6f/0x72
> > secondary_startup_64+0xa4/0xb0
> >
> > -> #1 (&p->pi_lock){-.-.}:
> > lock_acquire+0x9e/0x180
> > _raw_spin_lock_irqsave+0x3a/0x50
> > try_to_wake_up+0x41/0x600
> > wake_up_process+0x15/0x20
> > create_worker+0x16b/0x1e0
> > workqueue_init+0x279/0x2ee
> > kernel_init_freeable+0xf7/0x288
> > kernel_init+0xf/0x180
> > ret_from_fork+0x24/0x30
> >
> > -> #0 (&(&pool->lock)->rlock){-.-.}:
> > __lock_acquire+0x101f/0x12a0
> > lock_acquire+0x9e/0x180
> > _raw_spin_lock+0x2f/0x40
> > __queue_work+0xb2/0x520
> > queue_work_on+0x38/0x80
> > free_percpu+0x221/0x260
> > pcpu_freelist_destroy+0x11/0x20
> > stack_map_free+0x2a/0x40
> > bpf_map_free_deferred+0x3c/0x50
> > process_one_work+0x1f7/0x580
> > worker_thread+0x54/0x410
> > kthread+0x10f/0x150
> > ret_from_fork+0x24/0x30
> >
> > other info that might help us debug this:
> >
> > Chain exists of:
> > &(&pool->lock)->rlock --> &htab->buckets[i].lock --> pcpu_lock
> >
> > Possible unsafe locking scenario:
> >
> > CPU0 CPU1
> > ---- ----
> > lock(pcpu_lock);
> > lock(&htab->buckets[i].lock);
> > lock(pcpu_lock);
> > lock(&(&pool->lock)->rlock);
> >
> > *** DEADLOCK ***
> >
> > 3 locks held by kworker/23:255/18872:
> > #0: 00000000b36a6e16 ((wq_completion)events){+.+.},
> > at: process_one_work+0x17a/0x580
> > #1: 00000000dfd966f0 ((work_completion)(&map->work)){+.+.},
> > at: process_one_work+0x17a/0x580
> > #2: 00000000e3e7a6aa (pcpu_lock){..-.},
> > at: free_percpu+0x36/0x260
> >
> > stack backtrace:
> > CPU: 23 PID: 18872 Comm: kworker/23:255 Not tainted 5.1.0-dbg-DEV #1
> > Hardware name: ...
> > Workqueue: events bpf_map_free_deferred
> > Call Trace:
> > dump_stack+0x67/0x95
> > print_circular_bug.isra.38+0x1c6/0x220
> > check_prev_add.constprop.50+0x9f6/0xd20
> > __lock_acquire+0x101f/0x12a0
> > lock_acquire+0x9e/0x180
> > _raw_spin_lock+0x2f/0x40
> > __queue_work+0xb2/0x520
> > queue_work_on+0x38/0x80
> > free_percpu+0x221/0x260
> > pcpu_freelist_destroy+0x11/0x20
> > stack_map_free+0x2a/0x40
> > bpf_map_free_deferred+0x3c/0x50
> > process_one_work+0x1f7/0x580
> > worker_thread+0x54/0x410
> > kthread+0x10f/0x150
> > ret_from_fork+0x24/0x30
> >
> > Signed-off-by: John Sperbeck <jsperbeck@xxxxxxxxxx>
> > ---
> > mm/percpu.c | 6 +++++-
> > 1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index 68dd2e7e73b5..d832793bf83a 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -1738,6 +1738,7 @@ void free_percpu(void __percpu *ptr)
> > struct pcpu_chunk *chunk;
> > unsigned long flags;
> > int off;
> > + bool need_balance = false;
> >
> > if (!ptr)
> > return;
> > @@ -1759,7 +1760,7 @@ void free_percpu(void __percpu *ptr)
> >
> > list_for_each_entry(pos, &pcpu_slot[pcpu_nr_slots - 1], list)
> > if (pos != chunk) {
> > - pcpu_schedule_balance_work();
> > + need_balance = true;
> > break;
> > }
> > }
> > @@ -1767,6 +1768,9 @@ void free_percpu(void __percpu *ptr)
> > trace_percpu_free_percpu(chunk->base_addr, off, ptr);
> >
> > spin_unlock_irqrestore(&pcpu_lock, flags);
> > +
> > + if (need_balance)
> > + pcpu_schedule_balance_work();
> > }
> > EXPORT_SYMBOL_GPL(free_percpu);
> >
> > --
> > 2.21.0.1020.gf2820cf01a-goog
> >
>
> Hi John,
>
> The free_percpu() function hasn't changed in a little under 2 years. So,
> either lockdep has gotten smarter or something else has changed. There
> was a workqueue change recently merged: 6d25be5782e4 ("sched/core,
> workqueues: Distangle worker accounting from rq lock"). Would you mind
> reverting this and then seeing if you still encounter deadlock?
>

We have the issue even without 6d25be5782e4 in the picture.

I sent the splat months ago to Alexei, because I thought it was BPF
related at first