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([2601:188:c100:5710:627d:9ff:fe85:9ade]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4718cb6d2b0sm47264591cf.16.2025.02.12.08.45.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 12 Feb 2025 08:45:32 -0800 (PST) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: Date: Wed, 12 Feb 2025 11:45:31 -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 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. Cheers, Longman