> On Aug 27, 2024, at 9:21 PM, Paul E. McKenney <paul...@kernel.org> wrote: > > On Tue, Aug 27, 2024 at 11:33:32PM +0000, Jon Kohler wrote: >> >> >>> On Aug 27, 2024, at 4:34 PM, Paul E. McKenney <paul...@kernel.org> wrote: >>> >>> On Tue, Aug 27, 2024 at 08:01:27PM +0000, Jon Kohler wrote: >>>> Hey Paul, Lai, Josh, and the RCU list and Jan/FS list - >>>> Reaching out about a tricky hung task issue that I'm running into. I've >>>> got a virtualized Linux guest on top of a KVM based platform, running >>>> a 5.10.y based kernel. The issue we're running into is a hung task that >>>> *only* happens on shutdown/reboot of this particular VM once every >>>> 20-50 times. >>>> >>>> The signature of the hung task is always similar to the output below, >>>> where we appear to hang on the call to >>>> synchronize_srcu(&fsnotify_mark_srcu) >>> >>> One thing to try would be to add trace_printk() or similar to the SRCU >>> readers, just in case someone was using srcu_read_lock_notrace() on >>> fsnotify_mark_srcu, which I see no trace of in current mainline. >>> >>> Alternatively, if there is a version where this does not happen, try >>> bisecting. Each bisection step would require something like 400-500 >>> shutdown/reboots to prove the commit good. (Obviously, the first failure >>> proves the commit bad, which for one-out-of-50 failures will take on >>> average about 35 shutdown/reboots.) >>> >>> There could also be a bad SRCU backport from mainline, so please check >>> what SRCU backports you have in your 5.10.y stable release. (Though >>> maybe Jack has already done this?) >>> >>> Thanx, Paul >> >> Thanks, Paul >> >> For posterity, this kernel is just built off of the regular ole stable tree, >> and here’s >> All of the backports to kernel/rcu are below. >> >> Stepping through this more, since we’re stalling at wait_for_completion, >> that must mean that wakeme_after_rcu() ... complete(&rcu->completion) >> is not happening, right? >> >> That or somehow wakeme_after_rcu() fires *before* wait_for_completion() >> is setup, causing the wait to hang forever? > > There is no problem with that misordering -- in that case, the call to > wait_for_completion() simply won't wait. > > But that would be a good path to trace. After all, you might have > noticed that what we believe software will do does not always match what > it actually does.
Agreed, I’ll noodle on it some more, and try to bisect as well. I see Neeraj responded separately, I’ll check that out now. > > >> Is it possible for wakeme_after_rcu() to race, such that it fires somewhere >> else *before* wait_for_completion gets all the way to schedule()? > > That should also work just fine, but it is still a good assumption to > check. > > Good list below, but I must focus on mainline. Apologies! > > Plus this assumes that v5.10 worked for you -- has that been tested? > So again, what version has worked for you? We’ve had this particular service on 5.10.y for a while; however, this behavior just recently started bubbling up with a somewhat new(er) set of internal QA tests that very specifically stress shutdown/reboots in a loop for a completely unrelated durability test, so its possible this issue has existed for a while. > > Thanx, Paul > >> [rcu]$ pwd >> /kernel/kernel/rcu >> [rcu]$ git remote -v >> origin >> https://urldefense.proofpoint.com/v2/url?u=https-3A__git.kernel.org_pub_scm_linux_kernel_git_stable_linux.git&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=NGPRGGo37mQiSXgHKm5rCQ&m=1Tj05_-3u5bekAXIOn3TZBxlsLepxSWdhZVB6OKs02CYfxx4O9XkBJaihu2H2SaF&s=i-EJON2mPUDM-dvijeXkVr4nsR1g_obFaq9F9DCfQNs&e= >> (fetch) >> origin >> https://urldefense.proofpoint.com/v2/url?u=https-3A__git.kernel.org_pub_scm_linux_kernel_git_stable_linux.git&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=NGPRGGo37mQiSXgHKm5rCQ&m=1Tj05_-3u5bekAXIOn3TZBxlsLepxSWdhZVB6OKs02CYfxx4O9XkBJaihu2H2SaF&s=i-EJON2mPUDM-dvijeXkVr4nsR1g_obFaq9F9DCfQNs&e= >> (push) >> [rcu]$ git log --oneline 2c85ebc57b3e..HEAD . >> ca4427ebc626 (HEAD, tag: v5.10.205) Linux 5.10.205 <<<< this is the base >> commit for 5.10.205 >>>> >> ... >> 175f4b062f69 rcu: kmemleak: Ignore kmemleak false positives when RCU-freeing >> objects <<<< this is the most recent backport commit to kernel/rcu >>>> >> 55887adc76e1 rcuscale: Move rcu_scale_writer() >> schedule_timeout_uninterruptible() to _idle() >> 066fbd8bc981 refscale: Fix uninitalized use of wait_queue_head_t >> d93ba6e46e5f rcu-tasks: Add trc_inspect_reader() checks for exiting critical >> section >> 3e22624f8fd3 rcu-tasks: Wait for trc_read_check_handler() IPIs >> 9190c1f0aed1 rcu-tasks: Fix IPI failure handling in trc_wait_for_one_reader >> ad4f8c117b8b rcu: Prevent expedited GP from enabling tick on offline CPU >> 4f91de9a81bd rcu-tasks: Simplify trc_read_check_handler() atomic operations >> 3a64cd01cdd6 rcu-tasks: Mark ->trc_reader_special.b.need_qs data races >> 058f077d09ba rcu-tasks: Mark ->trc_reader_nesting data races >> 604d6a5ff718 rcu/rcuscale: Stop kfree_scale_thread thread(s) after unloading >> rcuscale >> d414e24d1509 rcu/rcuscale: Move rcu_scale_*() after kfree_scale_cleanup() >> ecc5e6dbc269 rcuscale: Move shutdown from wait_event() to wait_event_idle() >> b62c816bdb5e rcuscale: Always log error message >> 8cd9917c13a7 rcuscale: Console output claims too few grace periods >> 7230a9e599d3 rcu/kvfree: Avoid freeing new kfree_rcu() memory after old >> grace period >> a7d21b858589 rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access >> e4842de4ec13 refscale: Move shutdown from wait_event() to wait_event_idle() >> eb18bc5a8678 rcu: Avoid stack overflow due to __rcu_irq_enter_check_tick() >> being kprobe-ed >> d99d194e2f8c rcu-tasks: Make rude RCU-Tasks work well with CPU hotplug >> 2bf501f1bc78 rcu: Suppress smp_processor_id() complaint in >> synchronize_rcu_expedited_wait() >> 1c37e86a78c2 rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes() >> ad410f64f7ab rcu-tasks: Remove preemption disablement around >> srcu_read_[un]lock() calls >> b02b6bb83c68 rcu-tasks: Improve comments explaining tasks_rcu_exit_srcu >> purpose >> 7c15d7ecce00 rcu: Prevent lockdep-RCU splats on lock acquisition/release >> 5a52380b8193 rcu: Fix __this_cpu_read() lockdep warning in >> rcu_force_quiescent_state() >> 0dd025483f15 rcu-tasks: Convert RCU_LOCKDEP_WARN() to WARN_ONCE() >> 36d4ffbedff7 rcu: Back off upon fill_page_cache_func() allocation failure >> 10f30cba8f6c rcu: Make TASKS_RUDE_RCU select IRQ_WORK >> 1c6c3f233664 rcu-tasks: Fix race in schedule and flush work >> a22d66eb518f rcu: Apply callbacks processing time limit only on softirq >> 40fb3812d997 rcu: Fix callbacks processing time limit retaining >> cond_resched() >> fcc9797d0d13 rcu: Don't deboost before reporting expedited quiescent state >> 0c145262ac99 rcu/nocb: Fix missed nocb_timer requeue >> 657991fb06a4 rcu: Do not report strict GPs for outgoing CPUs >> 12d3389b7af6 rcu: Tighten rcu_advance_cbs_nowake() checks >> 0836f9404017 rcu/exp: Mark current CPU as exp-QS in IPI loop second pass >> 70692b06208c rcu: Mark accesses to rcu_state.n_force_qs >> af756be29c82 rcu: Always inline rcu_dynticks_task*_{enter,exit}() >> 226d68fb6c0a rcu: Fix existing exp request check in >> sync_sched_exp_online_cleanup() >> 02ddf26d849d rcu-tasks: Move RTGS_WAIT_CBS to beginning of >> rcu_tasks_kthread() loop >> 7f43cda650d5 rcutorture: Avoid problematic critical section nesting on >> PREEMPT_RT >> d3ca78775db4 rcu: Fix macro name CONFIG_TASKS_RCU_TRACE >> 497f3d9c3f58 rcu: Fix stall-warning deadlock due to non-release of rcu_node >> ->lock >> ea5e5bc881a4 rcu: Add lockdep_assert_irqs_disabled() to >> rcu_sched_clock_irq() and callees >> 527b56d7856f rcu: Fix to include first blocked task in stall warning >> 4b680b3fc6f3 rcu/tree: Handle VM stoppage in stall detection >> b6ae3854075e srcu: Provide polling interfaces for Tiny SRCU grace periods >> 450948b06ce8 srcu: Make Tiny SRCU use multi-bit grace-period counter >> 641e1d88404a srcu: Provide internal interface to start a Tiny SRCU grace >> period >> f789de3be808 srcu: Provide polling interfaces for Tree SRCU grace periods >> fdf66e5a7fc8 srcu: Provide internal interface to start a Tree SRCU grace >> period >> 86cb49e7314e rcu-tasks: Don't delete holdouts within >> trc_wait_for_one_reader() >> 55ddab2bfd70 rcu-tasks: Don't delete holdouts within trc_inspect_reader() >> 35a35909ec19 rcu: Reject RCU_LOCKDEP_WARN() false positives >> 23597afbe096 srcu: Fix broken node geometry after early ssp init >> 728f23e53c65 rcu: Invoke rcu_spawn_core_kthreads() from >> rcu_spawn_gp_kthread() >> 7d81aff28953 rcu: Remove spurious instrumentation_end() in rcu_nmi_enter() >> 09a27d662006 kvfree_rcu: Use same set of GFP flags as does single-argument >> e713bdd791ba rcu/nocb: Perform deferred wake up before last idle's >> need_resched() check >> 20b7669fa3f0 rcu: Pull deferred rcuog wake up to rcu_eqs_enter() callers >> 30b491e2b6cc rcu-tasks: Move RCU-tasks initialization to before >> early_initcall() >> 9b81af9c8455 rcu/tree: Defer kvfree_rcu() allocation to a clean context >> 5cacd18c5207 rcu,ftrace: Fix ftrace recursion >> 4540e84bd8a9 rcu: Allow rcu_irq_enter_check_tick() from NMI <<<< this is the >> first kernel/rcu backport for 5.10.y branch >>>> >> ... >> 2c85ebc57b3e (tag: v5.10) Linux 5.10 <<<< this is the divergence point from >> mainline to 5.10.y >>>> >> >>> >>>> in fsnotify_connector_destroy_workfn / fsnotify_mark_destroy_workfn, >>>> where two kernel threads are both calling synchronize_srcu, then >>>> scheduling out in wait_for_completion, and completely going out to >>>> lunch for over 4 minutes. This then triggers the hung task timeout and >>>> things blow up. >>>> >>>> We are running audit=1 for this system and are using an el8 based >>>> userspace. >>>> >>>> I've flipped through the fs/notify code base for both 5.10 as well as >>>> upstream mainline to see if something jumped off the page, and I >>>> haven't yet spotted any particular suspect code from the caller side. >>>> >>>> This hang appears to come up at the very end of the shutdown/reboot >>>> process, seemingly after the system starts to unwind through initrd. >>>> >>>> What I'm working on now is adding some instrumentation to the dracut >>>> shutdown initrd scripts to see if I can how far we get down that path >>>> before the system fails to make forward progress, which may give some >>>> hints. TBD on that. I've also enabled lockdep with CONFIG_PROVE_RCU and >>>> a plethora of DEBUG options [2], and didn't get anything interesting. >>>> To be clear, we haven't seen lockdep spit out any complaints as of yet. >>>> >>>> Reaching out to see if this sounds familar to anyone on the list, or if >>>> there are any particular areas of the RCU code base that might be >>>> suspect for this kind of issue. I'm happy to provide more information, >>>> as frankly, I'm quite stumped at the moment. >>>> >>>> Thanks all, >>>> Jon >>>> >>>> [1] panic trace >>>> Normal shutdown process, then hangs on the following: >>>> ... >>>> dracut Warning: Killing all remaining processes >>>> ... >>>> INFO: task kworker/u20:7:1200701 blocked for more than 241 seconds. >>>> Tainted: G O 5.10.205-2.el8.x86_64 #1 >>>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. >>>> task:kworker/u20:7 state:D stack: 0 pid:1200701 ppid: 2 >>>> flags:0x00004080 >>>> Workqueue: events_unbound fsnotify_connector_destroy_workfn >>>> Call Trace: >>>> __schedule+0x267/0x790 >>>> schedule+0x3c/0xb0 >>>> schedule_timeout+0x219/0x2b0 >>>> wait_for_completion+0x9e/0x100 >>>> __synchronize_srcu.part.24+0x83/0xb0 >>>> ? __bpf_trace_rcu_utilization+0x10/0x10 >>>> ? synchronize_srcu+0x5d/0xf0 >>>> fsnotify_connector_destroy_workfn+0x46/0x80 >>>> process_one_work+0x1fc/0x390 >>>> worker_thread+0x2d/0x3e0 >>>> ? process_one_work+0x390/0x390 >>>> kthread+0x114/0x130 >>>> ? kthread_park+0x80/0x80 >>>> ret_from_fork+0x1f/0x30 >>>> INFO: task kworker/u20:8:1287360 blocked for more than 241 seconds. >>>> Tainted: G O 5.10.205-2.el8.x86_64 #1 >>>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. >>>> task:kworker/u20:8 state:D stack: 0 pid:1287360 ppid: 2 >>>> flags:0x00004080 >>>> Workqueue: events_unbound fsnotify_mark_destroy_workfn >>>> Call Trace: >>>> __schedule+0x267/0x790 >>>> schedule+0x3c/0xb0 >>>> schedule_timeout+0x219/0x2b0 >>>> ? add_timer+0x14a/0x200 >>>> wait_for_completion+0x9e/0x100 >>>> __synchronize_srcu.part.24+0x83/0xb0 >>>> ? __bpf_trace_rcu_utilization+0x10/0x10 >>>> fsnotify_mark_destroy_workfn+0x77/0xe0 >>>> process_one_work+0x1fc/0x390 >>>> ? process_one_work+0x390/0x390 >>>> worker_thread+0x2d/0x3e0 >>>> ? process_one_work+0x390/0x390 >>>> kthread+0x114/0x130 >>>> ? kthread_park+0x80/0x80 >>>> ret_from_fork+0x1f/0x30 >>>> Kernel panic - not syncing: hung_task: blocked tasks >>>> CPU: 1 PID: 64 Comm: khungtaskd Kdump: loaded Tainted: G O >>>> 5.10.205-2.el8.x86_64 #1 >>>> Hardware name: Red Hat KVM, BIOS 20230302.1.2662.el8 04/01/2014 >>>> Call Trace: >>>> dump_stack+0x6d/0x8c >>>> panic+0x114/0x2ea >>>> watchdog.cold.8+0xb5/0xb5 >>>> ? hungtask_pm_notify+0x50/0x50 >>>> kthread+0x114/0x130 >>>> ? kthread_park+0x80/0x80 >>>> ret_from_fork+0x1f/0x30 >>>> >>>> [2] additional debugging config knobs turned up. >>>> CONFIG_PROVE_LOCKING=y >>>> CONFIG_LOCK_STAT=y >>>> CONFIG_DEBUG_RT_MUTEXES=y >>>> CONFIG_DEBUG_SPINLOCK=y >>>> CONFIG_DEBUG_MUTEXES=y >>>> CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y >>>> CONFIG_DEBUG_RWSEMS=y >>>> CONFIG_DEBUG_LOCK_ALLOC=y >>>> CONFIG_LOCKDEP=y >>>> CONFIG_LOCKDEP_BITS=15 >>>> CONFIG_LOCKDEP_CHAINS_BITS=16 >>>> CONFIG_LOCKDEP_STACK_TRACE_BITS=19 >>>> CONFIG_LOCKDEP_STACK_TRACE_HASH_BITS=14 >>>> CONFIG_LOCKDEP_CIRCULAR_QUEUE_BITS=12 >>>> CONFIG_DEBUG_SHIRQ=y >>>> CONFIG_WQ_WATCHDOG=y >>>> CONFIG_DEBUG_ATOMIC_SLEEP=y >>>> CONFIG_DEBUG_LIST=y >>>> CONFIG_DEBUG_PLIST=y >>>> CONFIG_DEBUG_SG=y >>>> CONFIG_DEBUG_NOTIFIERS=y >>>> CONFIG_BUG_ON_DATA_CORRUPTION=y
