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Date: Wed, 25 Jun 2014 11:42:31 -0400 From: Sasha Levin <sasha.levin@...cle.com> To: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>, peterz@...radead.org, tglx@...utronix.de, mingo@...nel.org, tj@...nel.org, rusty@...tcorp.com.au, akpm@...ux-foundation.org, fweisbec@...il.com, hch@...radead.org CC: mgorman@...e.de, riel@...hat.com, bp@...e.de, rostedt@...dmis.org, mgalbraith@...e.de, ego@...ux.vnet.ibm.com, paulmck@...ux.vnet.ibm.com, oleg@...hat.com, rjw@...ysocki.net, linux-kernel@...r.kernel.org, Dave Jones <davej@...hat.com> Subject: Re: [PATCH v7 2/2] CPU hotplug, smp: Flush any pending IPI callbacks before CPU offline 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@...il.com> > Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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