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[103.168.172.201]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6e65c3309d0sm154756d6.7.2025.02.12.09.09.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Feb 2025 09:09:28 -0800 (PST) Received: from phl-compute-03.internal (phl-compute-03.phl.internal [10.202.2.43]) by mailfauth.phl.internal (Postfix) with ESMTP id 0E65B120007C; Wed, 12 Feb 2025 12:09:28 -0500 (EST) Received: from phl-mailfrontend-01 ([10.202.2.162]) by phl-compute-03.internal (MEProxy); Wed, 12 Feb 2025 12:09:28 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefvddrtddtgdeggeeggecutefuodetggdotefrod ftvfcurfhrohhfihhlvgemucfhrghsthforghilhdpggftfghnshhusghstghrihgsvgdp uffrtefokffrpgfnqfghnecuuegrihhlohhuthemuceftddtnecusecvtfgvtghiphhivg hnthhsucdlqddutddtmdenucfjughrpeffhffvvefukfhfgggtugfgjgesthekredttddt udenucfhrhhomhepuehoqhhunhcuhfgvnhhguceosghoqhhunhdrfhgvnhhgsehgmhgrih hlrdgtohhmqeenucggtffrrghtthgvrhhnpedtgeehleevffdujeffgedvlefghffhleek ieeifeegveetjedvgeevueffieehhfenucevlhhushhtvghrufhiiigvpedtnecurfgrrh grmhepmhgrihhlfhhrohhmpegsohhquhhnodhmvghsmhhtphgruhhthhhpvghrshhonhgr lhhithihqdeiledvgeehtdeigedqudejjeekheehhedvqdgsohhquhhnrdhfvghngheppe hgmhgrihhlrdgtohhmsehfihigmhgvrdhnrghmvgdpnhgspghrtghpthhtohepiedpmhho uggvpehsmhhtphhouhhtpdhrtghpthhtoheplhhlohhnghesrhgvughhrghtrdgtohhmpd hrtghpthhtohepphgvthgvrhiisehinhhfrhgruggvrggurdhorhhgpdhrtghpthhtohep mhhinhhgohesrhgvughhrghtrdgtohhmpdhrtghpthhtohepfihilhhlrdguvggrtghonh esrghrmhdrtghomhdprhgtphhtthhopehlihhnuhigqdhkvghrnhgvlhesvhhgvghrrdhk vghrnhgvlhdrohhrghdprhgtphhtthhopegsohhquhhnsehfihigmhgvrdhnrghmvg X-ME-Proxy: Feedback-ID: iad51458e:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Wed, 12 Feb 2025 12:09:27 -0500 (EST) Date: Wed, 12 Feb 2025 09:09:26 -0800 From: Boqun Feng To: Waiman Long Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] locking/semaphore: Use wake_q to wake up processes outside lock critical section Message-ID: References: <20250127013127.3913153-1-longman@redhat.com> <3e45144d-d147-4431-91be-63d0817fa2ce@redhat.com> <5f518ef0-2dbc-4d14-82ce-ad310a780598@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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! Regards, Boqun > Cheers, > Longman >