linux-kernel - Re: [PATCH v3] locking/semaphore: Use wake_q to wake up processes outside lock critical section

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Message-ID: <Z6w1cvz29NkMZh9b@Mac.home>
Date: Tue, 11 Feb 2025 21:45:22 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Waiman Long <llong@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] locking/semaphore: Use wake_q to wake up processes
 outside lock critical section

On Tue, Feb 11, 2025 at 09:18:56PM -0500, Waiman Long wrote:
> 
> On 1/26/25 8:31 PM, Waiman Long wrote:
> > A circular lock dependency splat has been seen involving down_trylock().
> > 
> > [ 4011.795602] ======================================================
> > [ 4011.795603] WARNING: possible circular locking dependency detected
> > [ 4011.795607] 6.12.0-41.el10.s390x+debug
> > [ 4011.795612] ------------------------------------------------------
> > [ 4011.795613] dd/32479 is trying to acquire lock:
> > [ 4011.795617] 0015a20accd0d4f8 ((console_sem).lock){-.-.}-{2:2}, at: down_trylock+0x26/0x90
> > [ 4011.795636]
> > [ 4011.795636] but task is already holding lock:
> > [ 4011.795637] 000000017e461698 (&zone->lock){-.-.}-{2:2}, at: rmqueue_bulk+0xac/0x8f0
> > 
> >    the existing dependency chain (in reverse order) is:
> >    -> #4 (&zone->lock){-.-.}-{2:2}:
> >    -> #3 (hrtimer_bases.lock){-.-.}-{2:2}:
> >    -> #2 (&rq->__lock){-.-.}-{2:2}:
> >    -> #1 (&p->pi_lock){-.-.}-{2:2}:
> >    -> #0 ((console_sem).lock){-.-.}-{2:2}:
> > 
> > The console_sem -> pi_lock dependency is due to calling try_to_wake_up()
> > while holding the console.sem raw_spinlock. This dependency can be broken
> > by using wake_q to do the wakeup instead of calling try_to_wake_up()
> > under the console_sem lock. This will also make the semaphore's
> > raw_spinlock become a terminal lock without taking any further locks
> > underneath it.
> > 
> > The hrtimer_bases.lock is a raw_spinlock while zone->lock is a
> > spinlock. The hrtimer_bases.lock -> zone->lock dependency happens via
> > the debug_objects_fill_pool() helper function in the debugobjects code.
> > 
> > [ 4011.795646] -> #4 (&zone->lock){-.-.}-{2:2}:
> > [ 4011.795650]        __lock_acquire+0xe86/0x1cc0
> > [ 4011.795655]        lock_acquire.part.0+0x258/0x630
> > [ 4011.795657]        lock_acquire+0xb8/0xe0
> > [ 4011.795659]        _raw_spin_lock_irqsave+0xb4/0x120
> > [ 4011.795663]        rmqueue_bulk+0xac/0x8f0
> > [ 4011.795665]        __rmqueue_pcplist+0x580/0x830
> > [ 4011.795667]        rmqueue_pcplist+0xfc/0x470
> > [ 4011.795669]        rmqueue.isra.0+0xdec/0x11b0
> > [ 4011.795671]        get_page_from_freelist+0x2ee/0xeb0
> > [ 4011.795673]        __alloc_pages_noprof+0x2c2/0x520
> > [ 4011.795676]        alloc_pages_mpol_noprof+0x1fc/0x4d0
> > [ 4011.795681]        alloc_pages_noprof+0x8c/0xe0
> > [ 4011.795684]        allocate_slab+0x320/0x460
> > [ 4011.795686]        ___slab_alloc+0xa58/0x12b0
> > [ 4011.795688]        __slab_alloc.isra.0+0x42/0x60
> > [ 4011.795690]        kmem_cache_alloc_noprof+0x304/0x350
> > [ 4011.795692]        fill_pool+0xf6/0x450
> > [ 4011.795697]        debug_object_activate+0xfe/0x360
> > [ 4011.795700]        enqueue_hrtimer+0x34/0x190
> > [ 4011.795703]        __run_hrtimer+0x3c8/0x4c0
> > [ 4011.795705]        __hrtimer_run_queues+0x1b2/0x260
> > [ 4011.795707]        hrtimer_interrupt+0x316/0x760
> > [ 4011.795709]        do_IRQ+0x9a/0xe0
> > [ 4011.795712]        do_irq_async+0xf6/0x160
> > 
> > Normally raw_spinlock to spinlock dependency is not legit
> > and will be warned if PROVE_RAW_LOCK_NESTING is enabled,
> > but debug_objects_fill_pool() is an exception as it explicitly
> > allows this dependency for non-PREEMPT_RT kernel without causing
> > PROVE_RAW_LOCK_NESTING lockdep splat. As a result, this dependency is
> > legit and not a bug.
> > 
> > Anyway, semaphore is the only locking primitive left that is still
> > using try_to_wake_up() to do wakeup inside critical section, all the
> > other locking primitives had been migrated to use wake_q to do wakeup
> > outside of the critical section. It is also possible that there are
> > other circular locking dependencies involving printk/console_sem or
> > other existing/new semaphores lurking somewhere which may show up in
> > the future. Let just do the migration now to wake_q to avoid headache
> > like this.
> 
> I can also add the following as another instance where deadlock can happen.
> 
> Reported-by:syzbot+ed801a886dfdbfe7136d@...kaller.appspotmail.com
> 

FWIW, I already queued in my lockdep-for-tip branch, will send it in a
PR to Peter in one or two weeks (in case he hasn't taken it before
then).

BTW, do we need a "Fixes" tag for stable kernels?

Regards,
Boqun

> Cheers,
> Longman
> 
[...]