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[103.168.172.201]) by smtp.gmail.com with ESMTPSA id 19-20020a05620a04d300b0078838c7acbfsm2810654qks.42.2024.03.07.10.50.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 07 Mar 2024 10:50:49 -0800 (PST) Received: from compute5.internal (compute5.nyi.internal [10.202.2.45]) by mailfauth.nyi.internal (Postfix) with ESMTP id 9FB3B1200043; Thu, 7 Mar 2024 13:50:48 -0500 (EST) Received: from mailfrontend1 ([10.202.2.162]) by compute5.internal (MEProxy); Thu, 07 Mar 2024 13:50:48 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgedvledrieefgdduudejucetufdoteggodetrfdotf fvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfqfgfvpdfurfetoffkrfgpnffqhgen uceurghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmne cujfgurhepfffhvfevuffkfhggtggujgesthdtredttddtvdenucfhrhhomhepuehoqhhu nhcuhfgvnhhguceosghoqhhunhdrfhgvnhhgsehgmhgrihhlrdgtohhmqeenucggtffrrg htthgvrhhnpeehudfgudffffetuedtvdehueevledvhfelleeivedtgeeuhfegueeviedu ffeivdenucevlhhushhtvghrufhiiigvpedtnecurfgrrhgrmhepmhgrihhlfhhrohhmpe gsohhquhhnodhmvghsmhhtphgruhhthhhpvghrshhonhgrlhhithihqdeiledvgeehtdei gedqudejjeekheehhedvqdgsohhquhhnrdhfvghngheppehgmhgrihhlrdgtohhmsehfih igmhgvrdhnrghmvg X-ME-Proxy: Feedback-ID: iad51458e:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Thu, 7 Mar 2024 13:50:47 -0500 (EST) Date: Thu, 7 Mar 2024 10:49:41 -0800 From: Boqun Feng To: Stefan Wiehler Cc: Russell King , "Paul E. McKenney" , Joel Fernandes , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, Frederic Weisbecker Subject: Re: [PATCH] arm: smp: Avoid false positive CPU hotplug Lockdep-RCU splat Message-ID: References: <20240307160951.3607374-1-stefan.wiehler@nokia.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=us-ascii Content-Disposition: inline In-Reply-To: <20240307160951.3607374-1-stefan.wiehler@nokia.com> [Cc Frederic] On Thu, Mar 07, 2024 at 05:09:51PM +0100, Stefan Wiehler wrote: > With CONFIG_PROVE_RCU_LIST=y and by executing > > $ echo 0 > /sys/devices/system/cpu/cpu1/online > > one can trigger the following Lockdep-RCU splat on ARM: > > ============================= > WARNING: suspicious RCU usage > 6.8.0-rc7-00001-g0db1d0ed8958 #10 Not tainted > ----------------------------- > kernel/locking/lockdep.c:3762 RCU-list traversed in non-reader section!! > > other info that might help us debug this: > > RCU used illegally from offline CPU! > rcu_scheduler_active = 2, debug_locks = 1 > no locks held by swapper/1/0. > > stack backtrace: > CPU: 1 PID: 0 Comm: swapper/1 Not tainted 6.8.0-rc7-00001-g0db1d0ed8958 #10 > Hardware name: Allwinner sun8i Family > unwind_backtrace from show_stack+0x10/0x14 > show_stack from dump_stack_lvl+0x60/0x90 > dump_stack_lvl from lockdep_rcu_suspicious+0x150/0x1a0 > lockdep_rcu_suspicious from __lock_acquire+0x11fc/0x29f8 > __lock_acquire from lock_acquire+0x10c/0x348 > lock_acquire from _raw_spin_lock_irqsave+0x50/0x6c > _raw_spin_lock_irqsave from check_and_switch_context+0x7c/0x4a8 > check_and_switch_context from arch_cpu_idle_dead+0x10/0x7c > arch_cpu_idle_dead from do_idle+0xbc/0x138 > do_idle from cpu_startup_entry+0x28/0x2c > cpu_startup_entry from secondary_start_kernel+0x11c/0x124 > secondary_start_kernel from 0x401018a0 > > The CPU is already reported as offline from RCU perspective in > cpuhp_report_idle_dead() before arch_cpu_idle_dead() is invoked. Above > RCU-Lockdep splat is then triggered by check_and_switch_context() acquiring the > ASID spinlock. > > Avoid the false-positive Lockdep-RCU splat by briefly reporting the CPU as I'm not sure this is a false-positive: if you traverse an RCU-list without RCU watching the CPU, then a grace period may pass, the next list can be garbage and you can go anywhere from the list. Or am I missing something that a CPU hotplug can block a grace period? But codewise, it looks good to me. Although I hope we can avoid calling rcutree_report_cpu_{starting,dead}() here and use something simple, however after a few attempts, I don't find a better way. Regards, Boqun > online again while the spinlock is held. > > Signed-off-by: Stefan Wiehler > --- > arch/arm/kernel/smp.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c > index 3431c0553f45..6875e2c5dd50 100644 > --- a/arch/arm/kernel/smp.c > +++ b/arch/arm/kernel/smp.c > @@ -319,7 +319,14 @@ void __noreturn arch_cpu_idle_dead(void) > { > unsigned int cpu = smp_processor_id(); > > + /* > + * Briefly report CPU as online again to avoid false positive > + * Lockdep-RCU splat when check_and_switch_context() acquires ASID > + * spinlock. > + */ > + rcutree_report_cpu_starting(cpu); > idle_task_exit(); > + rcutree_report_cpu_dead(); > > local_irq_disable(); > > -- > 2.42.0 >