Re: Lockdep show 6.6-rc regression in Xen HVM CPU hotplug

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Tue, 2023-10-24 at 08:53 +0200, Juergen Gross wrote:
> 
> I'm puzzled. This path doesn't contain any of the RCU usage I've added in
> commit 87797fad6cce.
> 
> Are you sure that with just reverting commit 87797fad6cce the issue doesn't
> manifest anymore? I'd rather expect commit 721255b9826b having caused this
> behavior, just telling from the messages above.

Retesting in the cold light of day, yes. Using v6.6-rc5 which is the
parent commit of the offending 87797fad6cce.

I now see this warning at boot time again, which I believe was an
aspect of what you were trying to fix:

[    0.059014] xen:events: Using FIFO-based ABI
[    0.059029] xen:events: Xen HVM callback vector for event delivery is enabled
[    0.059227] rcu: srcu_init: Setting srcu_struct sizes based on contention.
[    0.059296] 
[    0.059297] =============================
[    0.059298] [ BUG: Invalid wait context ]
[    0.059299] 6.6.0-rc5 #1374 Not tainted
[    0.059300] -----------------------------
[    0.059301] swapper/0/0 is trying to lock:
[    0.059303] ffffffff8ad595f8 (evtchn_rwlock){....}-{3:3}, at: 
xen_evtchn_do_upcall+0x59/0xd0
[    0.059312] other info that might help us debug this:
[    0.059312] context-{2:2}
[    0.059313] no locks held by swapper/0/0.
[    0.059314] stack backtrace:
[    0.059315] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.6.0-rc5 #1374
[    0.059318] Hardware name: Xen HVM domU, BIOS 4.17.1 09/26/2023
[    0.059319] Call Trace:
[    0.059320]  <IRQ>
[    0.059323]  dump_stack_lvl+0x57/0x90
[    0.059327]  __lock_acquire+0x7bb/0xbb0
[    0.059334]  lock_acquire.part.0+0xad/0x240
[    0.059337]  ? xen_evtchn_do_upcall+0x59/0xd0
[    0.059340]  ? rcu_is_watching+0xd/0x40
[    0.059345]  ? lock_acquire+0xf2/0x110
[    0.059349]  _raw_read_lock+0x3b/0x70
[    0.059353]  ? xen_evtchn_do_upcall+0x59/0xd0
[    0.059355]  xen_evtchn_do_upcall+0x59/0xd0
[    0.059358]  __sysvec_xen_hvm_callback+0x47/0x80
[    0.059362]  sysvec_xen_hvm_callback+0x99/0xc0
[    0.059367]  </IRQ>
[    0.059368]  <TASK>
[    0.059369]  asm_sysvec_xen_hvm_callback+0x16/0x20
[    0.059372] RIP: 0010:__x86_return_thunk+0x0/0x30
[    0.059375] Code: 0b 66 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 
90 90 90 90 90 90 90 e9 3a ff ff ff 66 66 2e 0f 1f 84 00 00 00 00 00 <c3> cc 66 
66 2e 0f 1f 84 00 00 00 00 00 66 66 2e 0f 1f 84 00 00 00
[    0.059377] RSP: 0000:ffffffff8a803ef8 EFLAGS: 00000202
[    0.059379] RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000020
[    0.059381] RDX: 0000000000000000 RSI: ffffffff8a537ca9 RDI: ffffffff8a4cb0f7
[    0.059382] RBP: 00000000000000a0 R08: 0000000000000001 R09: 0000000000000000
[    0.059383] R10: 0000000000002e00 R11: ffff8fea0f7cd540 R12: 0000000000000000
[    0.059385] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000014010
[    0.059390]  start_kernel+0x26c/0x630
[    0.059396]  x86_64_start_reservations+0x14/0x30
[    0.059400]  x86_64_start_kernel+0x88/0x90
[    0.059403]  secondary_startup_64_no_verify+0x178/0x17b
[    0.059409]  </TASK>
[    0.171308] Console: colour VGA+ 80x25
[    0.171342] printk: console [hvc0] enabled

But offlining/onlining works without a problem:

[root@localhost cpu1]# echo 0 > online 
[  103.585558] smpboot: CPU 1 is now offline
[root@localhost cpu1]# echo 1 > online 
[  105.675214] installing Xen timer for CPU 1
[  105.675537] smpboot: Booting Node 0 Processor 1 APIC 0x2
[  105.677744] cpu 1 spinlock event irq 57
[root@localhost cpu1]# while true; do echo 0 > online ; echo 1 > online ; done

http://david.woodhou.se/config-hvm-hotplug-2023-10-24.txt

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