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([2601:188:c100:5710:627d:9ff:fe85:9ade]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6e44f4835e6sm54620516d6.111.2025.02.11.18.18.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 11 Feb 2025 18:18:57 -0800 (PST) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <3e45144d-d147-4431-91be-63d0817fa2ce@redhat.com> Date: Tue, 11 Feb 2025 21:18:56 -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: Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng Cc: linux-kernel@vger.kernel.org References: <20250127013127.3913153-1-longman@redhat.com> Content-Language: en-US In-Reply-To: <20250127013127.3913153-1-longman@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 Cheers, Longman > Signed-off-by: Waiman Long > --- > kernel/locking/semaphore.c | 13 +++++++++---- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c > index 34bfae72f295..de9117c0e671 100644 > --- a/kernel/locking/semaphore.c > +++ b/kernel/locking/semaphore.c > @@ -29,6 +29,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -38,7 +39,7 @@ static noinline void __down(struct semaphore *sem); > static noinline int __down_interruptible(struct semaphore *sem); > static noinline int __down_killable(struct semaphore *sem); > static noinline int __down_timeout(struct semaphore *sem, long timeout); > -static noinline void __up(struct semaphore *sem); > +static noinline void __up(struct semaphore *sem, struct wake_q_head *wake_q); > > /** > * down - acquire the semaphore > @@ -183,13 +184,16 @@ EXPORT_SYMBOL(down_timeout); > void __sched up(struct semaphore *sem) > { > unsigned long flags; > + DEFINE_WAKE_Q(wake_q); > > raw_spin_lock_irqsave(&sem->lock, flags); > if (likely(list_empty(&sem->wait_list))) > sem->count++; > else > - __up(sem); > + __up(sem, &wake_q); > raw_spin_unlock_irqrestore(&sem->lock, flags); > + if (!wake_q_empty(&wake_q)) > + wake_up_q(&wake_q); > } > EXPORT_SYMBOL(up); > > @@ -269,11 +273,12 @@ static noinline int __sched __down_timeout(struct semaphore *sem, long timeout) > return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout); > } > > -static noinline void __sched __up(struct semaphore *sem) > +static noinline void __sched __up(struct semaphore *sem, > + struct wake_q_head *wake_q) > { > struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list, > struct semaphore_waiter, list); > list_del(&waiter->list); > waiter->up = true; > - wake_up_process(waiter->task); > + wake_q_add(wake_q, waiter->task); > }