Oops in rapl_cpu_prepare()

["Charles (Chas) Williams" <ciwillia@brocade.com>]

Recent 4.8 kernels have been oopsing when running under VMWare:

[    2.270203] BUG: unable to handle kernel NULL pointer dereference at 0000000000000408
[    2.270325] IP: [<ffffffff81012bb9>] rapl_cpu_online+0x59/0x70
[    2.270448] PGD 0
[    2.270570] Oops: 0002 [#1] SMP
[    2.270693] Modules linked in:
[    2.270815] CPU: 2 PID: 21 Comm: cpuhp/2 Not tainted 4.8.2-1-amd64-vyatta #1
[    2.270938] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/14/2014
[    2.271060] task: ffff8802361fc2c0 task.stack: ffff880236208000
[    2.271183] RIP: 0010:[<ffffffff81012bb9>]  [<ffffffff81012bb9>] rapl_cpu_online+0x59/0x70
[    2.271306] RSP: 0000:ffff88023620be68  EFLAGS: 00010246
[    2.271428] RAX: 0000000000000004 RBX: ffff88023fd0d940 RCX: 0000000000000000
[    2.271551] RDX: 0000000000000040 RSI: 0000000000000004 RDI: 0000000000000004
[    2.271673] RBP: 0000000000000002 R08: fffffffffffffffc R09: 0000000000000000
[    2.271796] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000400
[    2.271918] R13: ffff8802361fc2c0 R14: ffff8802361fc2c0 R15: ffff8802361fc2c0
[    2.272041] FS:  0000000000000000(0000) GS:ffff88023fd00000(0000) knlGS:0000000000000000
[    2.272163] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    2.272286] CR2: 0000000000000408 CR3: 0000000001a06000 CR4: 00000000000406e0
[    2.272408] Stack:
[    2.272531]  ffff88023fd0d940 0000000000000002 ffffffff81a38240 ffffffff81061231
[    2.272654]  ffff8802361fc2c0 ffff880237002180 ffffffff8107ddcf 0000000000000000
[    2.272776]  ffff8802361a5a80 ffff880237002180 ffffffff8107dcb0 ffffffff81a6a380
[    2.272899] Call Trace:
[    2.273021]  [<ffffffff81061231>] ? cpuhp_thread_fun+0x31/0x100
[    2.273144]  [<ffffffff8107ddcf>] ? smpboot_thread_fn+0x11f/0x180
[    2.273266]  [<ffffffff8107dcb0>] ? sort_range+0x20/0x20
[    2.273389]  [<ffffffff8107b05a>] ? kthread+0xca/0xe0
[    2.273511]  [<ffffffff8157677f>] ? ret_from_fork+0x1f/0x40
[    2.273634]  [<ffffffff8107af90>] ? kthread_park+0x50/0x50
[    2.273757] Code: 00 00 48 83 c0 22 4c 8b 24 c1 48 c7 c0 30 a1 00 00 48 8b 14 10 e8 a8 61 26 00 3b 05 b6 56 ae 00 7c 0e f0 48 0f a
[    2.279445] RIP  [<ffffffff81012bb9>] rapl_cpu_online+0x59/0x70
[    2.279568]  RSP <ffff88023620be68>
[    2.279690] CR2: 0000000000000408
[    2.279813] ---[ end trace c95da920748eb432 ]---


gdb tells me:

(gdb) info line *(rapl_cpu_online+0x59)
Line 595 of "arch/x86/events/intel/rapl.c" starts at address 0xffffffff81012bb9 <rapl_cpu_online+89>
    and ends at 0xffffffff81012bbe <rapl_cpu_online+94>.

Which is:


         target = cpumask_any_and(&rapl_cpu_mask, topology_core_cpumask(cpu));
         if (target < nr_cpu_ids)
                 return 0;

         cpumask_set_cpu(cpu, &rapl_cpu_mask);
         pmu->cpu = cpu;		<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
         return 0;

This code was recently changed by commit 8b5b773d6245138c
"perf/x86/intel/rapl: Convert to hotplug state machine" and it
appears that the setup is done as a callback:

         /*
          * Install callbacks. Core will call them for each online cpu.
          */

         ret = cpuhp_setup_state(CPUHP_PERF_X86_RAPL_PREP, "PERF_X86_RAPL_PREP",
                                 rapl_cpu_prepare, NULL);
         if (ret)
                 goto out;

         ret = cpuhp_setup_state(CPUHP_AP_PERF_X86_RAPL_ONLINE,
                                 "AP_PERF_X86_RAPL_ONLINE",
                                 rapl_cpu_online, rapl_cpu_offline);

Is there a particular order guaranteed by the callbacks?  Will
rapl_cpu_prepare() always happen before online/offline?  Additionally,
rapl_cpu_prepare() can fail to allocate pmu,

	static int rapl_cpu_prepare(unsigned int cpu)
	{
		struct rapl_pmu *pmu = cpu_to_rapl_pmu(cpu);

		if (pmu)
			return 0;

		pmu = kzalloc_node(sizeof(*pmu), GFP_KERNEL, cpu_to_node(cpu));
		if (!pmu)
			return -ENOMEM;

But rapl_cpu_online() would have no idea about this.  What should be
done in this case?