Message-ID: <efbe0c96-2bc6-4520-a90d-57e273f9f613@redhat.com>
Date: Wed, 12 Feb 2025 11:45:31 -0500
From: Waiman Long <llong@...hat.com>
To: Boqun Feng <boqun.feng@...il.com>, 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 2/12/25 11:38 AM, Boqun Feng wrote:
> On Wed, Feb 12, 2025 at 09:10:25AM -0500, Waiman Long wrote:
>> On 2/12/25 12:45 AM, Boqun Feng wrote:
>>> 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?
>> After some more thought, I realize that this patch doesn't really fix the
>> circular lock dependency problem, it just remove console_sem.lock from it.
>> The problem is that printk() can be called in any context. To really solve
>> the problem, we will need some kind of deferred wakeup using workqueue, for
>> instance. As printing to the console is inherently slow, adding some more
> Hmm... but your patch does remove the dependency console_sem.lock ->
> pi_lock, right? So it does fix the circular lock dependency problem. Or
> do you mean that it doesn't fix all the deadlocks that may cause by
> printk()?

Right, it doesn't  fix all the deadlocks that may be caused by printk(). 
Similar circular lock dependency splat will still happen. It will start 
with pi_lock instead of console_sem.lock and will have one less lock in 
the circular list. It is caused by waking up process within the printk() 
context. That is why I think the proper solution is to have a deferred 
wakeup. It will be specific to the printk() use cases.

Cheers,
Longman

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