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([2601:188:c100:5710:627d:9ff:fe85:9ade]) by smtp.gmail.com with ESMTPSA id 586e51a60fabf-2b854f77c92sm4418051fac.30.2025.02.12.10.36.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 12 Feb 2025 10:36:27 -0800 (PST) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: Date: Wed, 12 Feb 2025 13:36:26 -0500 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] locking/semaphore: Use wake_q to wake up processes outside lock critical section To: Boqun Feng , Waiman Long Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , linux-kernel@vger.kernel.org References: <20250127013127.3913153-1-longman@redhat.com> <3e45144d-d147-4431-91be-63d0817fa2ce@redhat.com> <5f518ef0-2dbc-4d14-82ce-ad310a780598@redhat.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2/12/25 12:09 PM, Boqun Feng wrote: > On Wed, Feb 12, 2025 at 11:45:31AM -0500, Waiman Long wrote: >> 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@syzkaller.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. >> > Sounds good. Then I will take this patch, because 1) it fixes an issue > (although partially) and 2) the fix is toward the right direction (i.e. > avoiding wakeup inside critical section). Thanks! This patch does fix some deadlock cases. The process wakeup will happen in console_unlock(). Its comment says that "console_unlock(); may be called from any context". So there may still be other deadlock cases hidden somewhere. Cheers, Longman