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References: <20190508014320.55404-1-jsperbeck@google.com> <20190508185932.GA68786@dennisz-mbp>
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From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 8 May 2019 12:04:08 -0700
Message-ID: <CANn89iJa7qLqDjQOV9y_f3jsLogv9K0j1x=+eViKa2MQEcEjBw@mail.gmail.com>
Subject: Re: [PATCH] percpu: remove spurious lock dependency between percpu
 and sched
To:     Dennis Zhou <dennis@kernel.org>, Alexei Starovoitov <ast@fb.com>
Cc:     John Sperbeck <jsperbeck@google.com>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux.com>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, May 8, 2019 at 11:59 AM Dennis Zhou <dennis@kernel.org> 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@google.com>
> > ---
> >  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