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([2601:188:ca00:a00:f844:fad5:7984:7bd7]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7ae7563736esm265415985a.67.2024.10.07.08.23.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 07 Oct 2024 08:23:33 -0700 (PDT) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <95365aa5-d039-4d09-8191-516ba01c9e5d@redhat.com> Date: Mon, 7 Oct 2024 11:23:32 -0400 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] locking/rtmutex: Always use trylock in rt_mutex_trylock() To: Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Boqun Feng , linux-kernel@vger.kernel.org, Thomas Gleixner , Luis Goncalves , Chunyu Hu References: <20240926151315.507905-1-longman@redhat.com> <20241002093745.GO5594@noisy.programming.kicks-ass.net> <7918987a-4b66-4191-aa52-798f9434352a@redhat.com> <20241007145037.GE4879@noisy.programming.kicks-ass.net> Content-Language: en-US In-Reply-To: <20241007145037.GE4879@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10/7/24 10:50 AM, Peter Zijlstra wrote: > On Wed, Oct 02, 2024 at 01:54:16PM -0400, Waiman Long wrote: >> On 10/2/24 05:37, Peter Zijlstra wrote: >>> On Thu, Sep 26, 2024 at 11:13:15AM -0400, Waiman Long wrote: >>>> One reason to use a trylock is to avoid a ABBA deadlock in case we need >>>> to use a locking sequence that is not in the expected locking order. That >>>> requires the use of trylock all the ways in the abnormal locking >>>> sequence. Unfortunately, this is not the case for rt_mutex_trylock() as >>>> it uses a raw_spin_lock_irqsave() to acquire the lock->wait_lock. That >>>> will cause a lockdep splat like the following in a PREEMPT_RT kernel: >>>> >>>> [ 63.695668] -> #0 (&lock->wait_lock){-...}-{2:2}: >>>> [ 63.695674] check_prev_add+0x1bd/0x1310 >>>> [ 63.695678] validate_chain+0x6cf/0x7c0 >>>> [ 63.695682] __lock_acquire+0x879/0xf40 >>>> [ 63.695686] lock_acquire.part.0+0xfa/0x2d0 >>>> [ 63.695690] _raw_spin_lock_irqsave+0x46/0x90 >>>> [ 63.695695] rt_mutex_slowtrylock+0x3f/0xb0 >>>> [ 63.695699] rt_spin_trylock+0x13/0xc0 >>>> [ 63.695702] rmqueue_pcplist+0x5b/0x180 >>>> [ 63.695705] rmqueue+0xb01/0x1040 >>>> [ 63.695708] get_page_from_freelist+0x1d0/0xa00 >>>> [ 63.695712] __alloc_pages_noprof+0x19a/0x450 >>>> [ 63.695716] alloc_pages_mpol_noprof+0xaf/0x1e0 >>>> [ 63.695721] stack_depot_save_flags+0x4db/0x520 >>>> [ 63.695727] kasan_save_stack+0x3f/0x50 >>>> [ 63.695731] __kasan_record_aux_stack+0x8e/0xa0 >>>> [ 63.695736] task_work_add+0x1ad/0x240 >>>> [ 63.695741] sched_tick+0x1c7/0x500 >>>> [ 63.695744] update_process_times+0xf1/0x130 >>>> [ 63.695750] tick_nohz_handler+0xf7/0x230 >>>> [ 63.695754] __hrtimer_run_queues+0x13b/0x7b0 >>>> [ 63.695758] hrtimer_interrupt+0x1c2/0x350 >>>> [ 63.695763] __sysvec_apic_timer_interrupt+0xdb/0x340 >>>> [ 63.695770] sysvec_apic_timer_interrupt+0x9c/0xd0 >>>> [ 63.695774] asm_sysvec_apic_timer_interrupt+0x1a/0x20 >>>> [ 63.695780] __asan_load8+0x8/0xa0 >>>> [ 63.695784] mas_wr_end_piv+0x28/0x2c0 >>>> [ 63.695789] mas_preallocate+0x3a8/0x680 >>>> [ 63.695793] vma_shrink+0x180/0x3f0 >>>> [ 63.695799] shift_arg_pages+0x219/0x2c0 >>>> [ 63.695804] setup_arg_pages+0x343/0x5e0 >>>> [ 63.695807] load_elf_binary+0x5ac/0x15d0 >>>> [ 63.695813] search_binary_handler+0x125/0x370 >>>> [ 63.695818] exec_binprm+0xc9/0x3d0 >>>> [ 63.695821] bprm_execve+0x103/0x230 >>>> [ 63.695824] kernel_execve+0x187/0x1c0 >>>> [ 63.695828] call_usermodehelper_exec_async+0x145/0x1e0 >>>> [ 63.695832] ret_from_fork+0x31/0x60 >>>> [ 63.695836] ret_from_fork_asm+0x1a/0x30 >>>> [ 63.695840] >>>> [ 63.695840] other info that might help us debug this: >>>> [ 63.695840] >>>> [ 63.695842] Chain exists of: >>>> [ 63.695842] &lock->wait_lock --> &p->pi_lock --> &rq->__lock >>>> [ 63.695842] >>>> [ 63.695850] Possible unsafe locking scenario: >>>> [ 63.695850] >>>> [ 63.695851] CPU0 CPU1 >>>> [ 63.695852] ---- ---- >>>> [ 63.695854] lock(&rq->__lock); >>>> [ 63.695857] lock(&p->pi_lock); >>>> [ 63.695861] lock(&rq->__lock); >>>> [ 63.695864] lock(&lock->wait_lock); >>>> [ 63.695868] >>>> [ 63.695868] *** DEADLOCK *** >>>> >>>> Fix it by using raw_spin_trylock_irqsave() instead. >>> That truncated lockdep report doesn't really explain anything. Please >>> just transcribe the full lockdep report into something small and >>> coherent. >> I was trying to show where the offending call is coming from. I will send a >> v2 with a condensed lockdep splat. > No no no... explain the actual problem. > > Is the problem that: > > sched_tick() > task_tick_mm_cid() > task_work_add() > kasan_save_stack() > idiotic crap while holding rq->__lock ? > > Because afaict that is completely insane. And has nothing to do with > rtmutex. > > We are not going to change rtmutex because instrumentation shit is shit. Yes, it is because of KASAN that causes page allocation while holding the rq->__lock. Maybe we can blame KASAN for this. It is actually not a problem for non-PREEMPT_RT kernel because only trylock is being used. However, we don't use trylock all the way when rt_spin_trylock() is being used with PREEMPT_RT Kernel. This is certainly a problem that we need to fix as there may be other similar case not involving rq->__lock lurking somewhere. Cheers, Longman >