BUG: workqueue lockup - SRCU schedules work on not-online CPUs during size transition

[Vasily Gorbik <gor@linux.ibm.com>]

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