Messages in this thread |  | | Date | Thu, 9 Apr 2026 15:08:45 +0200 | | From | Vasily Gorbik <> | | Subject | BUG: workqueue lockup - SRCU schedules work on not-online CPUs during size transition |
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Commit 61bbcfb50514 ("srcu: Push srcu_node allocation to GP when non-preemptible") defers srcu_node tree allocation when called under raw spinlock, putting SRCU through ~6 transitional grace periods (SRCU_SIZE_ALLOC to SRCU_SIZE_BIG). During this transition srcu_gp_end() uses mask = ~0, which makes srcu_schedule_cbs_snp() call queue_work_on() for every possible CPU. Since rcu_gp_wq is WQ_PERCPU, work targets per-CPU pools directly - pools for not-online CPUs have no workers, work accumulates, workqueue lockup detector fires.
Before 61bbcfb50514, GFP_ATOMIC allocation went straight to SRCU_SIZE_BIG, the mask = ~0 path was never reached.
Affects systems with convert_to_big active (auto when nr_cpu_ids >= 128) and possible CPUs > online CPUs. Hit on s390 LPAR (76 online, 400 possible), where possible CPUs > online CPUs is the usual case. Also reproducible on x86 KVM --smp 16,maxcpus=255 (CONFIG_NR_CPUS=256) or simply -smp 1,maxcpus=2 with srcutree.convert_to_big=1 or --smp 16,maxcpus=64 with srcutree.big_cpu_lim=32 (CONFIG_NR_CPUS=64)
s390 log (76 online CPUs, 400 possible, all pools 76-399 stuck):
BUG: workqueue lockup - pool cpus=76 node=0 flags=0x4 nice=0 stuck for 1842s! BUG: workqueue lockup - pool cpus=77 node=0 flags=0x4 nice=0 stuck for 1842s! ... BUG: workqueue lockup - pool cpus=399 node=0 flags=0x4 nice=0 stuck for 1842s! Showing busy workqueues and worker pools: workqueue rcu_gp: flags=0x108 pwq 306: cpus=76 node=0 flags=0x4 nice=0 active=3 refcnt=4 pending: 3*srcu_invoke_callbacks pwq 310: cpus=77 node=0 flags=0x4 nice=0 active=3 refcnt=4 pending: 3*srcu_invoke_callbacks ... pwq 1598: cpus=399 node=0 flags=0x4 nice=0 active=3 refcnt=4 pending: 3*srcu_invoke_callbacks
Not sure if replacing mask = ~0 with something derived from cpu_online_mask would be racy in that context.
[1] https://lore.kernel.org/rcu/acRho9L4zA2MRuxc@tardis.local [2] https://lore.kernel.org/rcu/fe28d664-3872-40f6-83c6-818627ad5b7d@paulmck-laptop
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