Re: [PATCH v7 2/2] CPU hotplug, smp: Flush any pending IPI callbacks before CPU offline

[Sasha Levin]

On 05/26/2014 07:08 AM, Srivatsa S. Bhat wrote:
> During CPU offline, in stop-machine, we don't enforce any rule in the
> _DISABLE_IRQ stage, regarding the order in which the outgoing CPU and the other
> CPUs disable their local interrupts. Hence, we can encounter a scenario as
> depicted below, in which IPIs are sent by the other CPUs to the CPU going
> offline (while it is *still* online), but the outgoing CPU notices them only
> *after* it has gone offline.
> 
> 
>               CPU 1                                         CPU 2
>           (Online CPU)                               (CPU going offline)
> 
>        Enter _PREPARE stage                          Enter _PREPARE stage
> 
>                                                      Enter _DISABLE_IRQ stage
> 
> 
>                                                    =
>        Got a device interrupt,                     | Didn't notice the IPI
>        and the interrupt handler                   | since interrupts were
>        called smp_call_function()                  | disabled on this CPU.
>        and sent an IPI to CPU 2.                   |
>                                                    =
> 
> 
>        Enter _DISABLE_IRQ stage
> 
> 
>        Enter _RUN stage                              Enter _RUN stage
> 
>                                   =
>        Busy loop with interrupts  |                  Invoke take_cpu_down()
>        disabled.                  |                  and take CPU 2 offline
>                                   =
> 
> 
>        Enter _EXIT stage                             Enter _EXIT stage
> 
>        Re-enable interrupts                          Re-enable interrupts
> 
>                                                      The pending IPI is noted
>                                                      immediately, but alas,
>                                                      the CPU is offline at
>                                                      this point.
> 
> 
> 
> This of course, makes the smp-call-function IPI handler code unhappy and it
> complains about "receiving an IPI on an offline CPU".
> 
> However, if we look closely, we observe that the IPI was sent when CPU 2 was
> still online, and hence it was perfectly legal for CPU 1 to send the IPI at
> that point. Furthermore, receiving an IPI on an offline CPU is terrible only
> if there were pending callbacks yet to be executed by that CPU (in other words,
> its a bug if the CPU went offline with work still pending).
> 
> So, fix this by flushing all the queued smp-call-function callbacks on the
> outgoing CPU in the CPU_DYING stage[1], including those callbacks for which the
> source CPU's IPIs might not have been received on the outgoing CPU yet. This
> ensures that all pending IPI callbacks are run before the CPU goes completely
> offline. But note that the outgoing CPU can still get IPIs from the other CPUs
> just after it exits stop-machine, due to the scenario mentioned above. But
> because we flush the callbacks before going offline, this will be completely
> harmless.
> 
> Further, this solution also guarantees that there will be pending callbacks
> on an offline CPU *only if* the source CPU initiated the IPI-send-procedure
> *after* the target CPU went offline, which clearly indicates a bug in the
> sender code.
> 
> So, considering all this, teach the smp-call-function IPI handler code to
> complain only if an offline CPU received an IPI *and* it still had pending
> callbacks to execute, since that is the only buggy scenario.
> 
> There is another case (somewhat theoretical though) where IPIs might arrive
> late on the target CPU (possibly _after_ the CPU has gone offline): due to IPI
> latencies in the hardware. But with this patch, even this scenario turns out
> to be harmless, since we explicitly loop through the call_single_queue and
> flush out any pending callbacks without waiting for the corresponding IPIs
> to arrive.
> 
> 
> [1]. The CPU_DYING part needs a little more explanation: by the time we
> execute the CPU_DYING notifier callbacks, the CPU would have already been
> marked offline. But we want to flush out the pending callbacks at this stage,
> ignoring the fact that the CPU is offline. So restructure the IPI handler
> code so that we can by-pass the "is-cpu-offline?" check in this particular
> case. (Of course, the right solution here is to fix CPU hotplug to mark the
> CPU offline _after_ invoking the CPU_DYING notifiers, but this requires a
> lot of audit to ensure that this change doesn't break any existing code;
> hence lets go with the solution proposed above until that is done).
> 
> Suggested-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>

Hi all,

While fuzzing with trinity inside a KVM tools guest running the latest -next
kernel I've stumbled on the following spew:

[ 1982.600053] kernel BUG at kernel/irq_work.c:175!
[ 1982.600053] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 1982.600053] Dumping ftrace buffer:
[ 1982.600053]    (ftrace buffer empty)
[ 1982.600053] Modules linked in:
[ 1982.600053] CPU: 14 PID: 168 Comm: migration/14 Not tainted 3.16.0-rc2-next-20140624-sasha-00024-g332b58d #726
[ 1982.600053] task: ffff88036a5a3000 ti: ffff88036a5ac000 task.ti: ffff88036a5ac000
[ 1982.600053] RIP: irq_work_run (kernel/irq_work.c:175 (discriminator 1))
[ 1982.600053] RSP: 0000:ffff88036a5afbe0  EFLAGS: 00010046
[ 1982.600053] RAX: 0000000080000001 RBX: 0000000000000000 RCX: 0000000000000008
[ 1982.600053] RDX: 000000000000000e RSI: ffffffffaf9185fb RDI: 0000000000000000
[ 1982.600053] RBP: ffff88036a5afc08 R08: 0000000000099224 R09: 0000000000000000
[ 1982.600053] R10: 0000000000000000 R11: 0000000000000001 R12: ffff88036afd8400
[ 1982.600053] R13: 0000000000000000 R14: ffffffffb0cf8120 R15: ffffffffb0cce5d0
[ 1982.600053] FS:  0000000000000000(0000) GS:ffff88036ae00000(0000) knlGS:0000000000000000
[ 1982.600053] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 1982.600053] CR2: 00000000019485d0 CR3: 00000002c7c8f000 CR4: 00000000000006a0
[ 1982.600053] Stack:
[ 1982.600053]  ffffffffab20fbb5 0000000000000082 ffff88036afd8440 0000000000000000
[ 1982.600053]  0000000000000001 ffff88036a5afc28 ffffffffab20fca7 0000000000000000
[ 1982.600053]  00000000ffffffef ffff88036a5afc78 ffffffffab19c58e 000000000000000e
[ 1982.600053] Call Trace:
[ 1982.600053] ? flush_smp_call_function_queue (kernel/smp.c:263)
[ 1982.600053] hotplug_cfd (kernel/smp.c:81)
[ 1982.600053] notifier_call_chain (kernel/notifier.c:95)
[ 1982.600053] __raw_notifier_call_chain (kernel/notifier.c:395)
[ 1982.600053] __cpu_notify (kernel/cpu.c:202)
[ 1982.600053] cpu_notify (kernel/cpu.c:211)
[ 1982.600053] take_cpu_down (./arch/x86/include/asm/current.h:14 kernel/cpu.c:312)
[ 1982.600053] multi_cpu_stop (kernel/stop_machine.c:201)
[ 1982.600053] ? __stop_cpus (kernel/stop_machine.c:170)
[ 1982.600053] cpu_stopper_thread (kernel/stop_machine.c:474)
[ 1982.600053] ? put_lock_stats.isra.12 (./arch/x86/include/asm/preempt.h:98 kernel/locking/lockdep.c:254)
[ 1982.600053] ? _raw_spin_unlock_irqrestore (./arch/x86/include/asm/paravirt.h:809 include/linux/spinlock_api_smp.h:160 kernel/locking/spinlock.c:191)
[ 1982.600053] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
[ 1982.600053] ? trace_hardirqs_on_caller (kernel/locking/lockdep.c:2557 kernel/locking/lockdep.c:2599)
[ 1982.600053] smpboot_thread_fn (kernel/smpboot.c:160)
[ 1982.600053] ? __smpboot_create_thread (kernel/smpboot.c:105)
[ 1982.600053] kthread (kernel/kthread.c:210)
[ 1982.600053] ? wait_for_completion (kernel/sched/completion.c:77 kernel/sched/completion.c:93 kernel/sched/completion.c:101 kernel/sched/completion.c:122)
[ 1982.600053] ? kthread_create_on_node (kernel/kthread.c:176)
[ 1982.600053] ret_from_fork (arch/x86/kernel/entry_64.S:349)
[ 1982.600053] ? kthread_create_on_node (kernel/kthread.c:176)
[ 1982.600053] Code: 00 00 00 00 e8 63 ff ff ff 48 83 c4 08 b8 01 00 00 00 5b 5d c3 b8 01 00 00 00 c3 90 65 8b 04 25 a0 da 00 00 a9 00 00 0f 00 75 09 <0f> 0b 0f 1f 80 00 00 00 00 55 48 89 e5 e8 2f ff ff ff 5d c3 66
All code
========
   0:	00 00                	add    %al,(%rax)
   2:	00 00                	add    %al,(%rax)
   4:	e8 63 ff ff ff       	callq  0xffffffffffffff6c
   9:	48 83 c4 08          	add    $0x8,%rsp
   d:	b8 01 00 00 00       	mov    $0x1,%eax
  12:	5b                   	pop    %rbx
  13:	5d                   	pop    %rbp
  14:	c3                   	retq
  15:	b8 01 00 00 00       	mov    $0x1,%eax
  1a:	c3                   	retq
  1b:	90                   	nop
  1c:	65 8b 04 25 a0 da 00 	mov    %gs:0xdaa0,%eax
  23:	00
  24:	a9 00 00 0f 00       	test   $0xf0000,%eax
  29:	75 09                	jne    0x34
  2b:*	0f 0b                	ud2    		<-- trapping instruction
  2d:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)
  34:	55                   	push   %rbp
  35:	48 89 e5             	mov    %rsp,%rbp
  38:	e8 2f ff ff ff       	callq  0xffffffffffffff6c
  3d:	5d                   	pop    %rbp
  3e:	c3                   	retq
  3f:	66                   	data16
	...

Code starting with the faulting instruction
===========================================
   0:	0f 0b                	ud2
   2:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)
   9:	55                   	push   %rbp
   a:	48 89 e5             	mov    %rsp,%rbp
   d:	e8 2f ff ff ff       	callq  0xffffffffffffff41
  12:	5d                   	pop    %rbp
  13:	c3                   	retq
  14:	66                   	data16
	...
[ 1982.600053] RIP irq_work_run (kernel/irq_work.c:175 (discriminator 1))
[ 1982.600053]  RSP <ffff88036a5afbe0>


Thanks,
Sasha
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