Re: [PATCH stable 4.9] perf/x86/intel/rapl: Make package handling more robust

Re: [PATCH stable 4.9] perf/x86/intel/rapl: Make package handling more robust

[Jinpu Wang]

Sorry, this is a backport for stable 4.9, Greg please consider to
apply to 4.9, this fix NULL pointer deref during cpu hot-add in debian
9 VM.

[  270.876510] CPU10 has been hot-added
[  270.891832] smpboot: Booting Node 0 Processor 10 APIC 0xa
[  270.887997] kvm-clock: cpu 10, msr 1:3fff0281, secondary cpu clock
[  270.990904] TSC synchronization [CPU#4 -> CPU#10]:
[  270.990906] Measured 713133966959 cycles TSC warp between CPUs,
turning off TSC clock.
[  270.990910] tsc: Marking TSC unstable due to check_tsc_sync_source failed
[  270.990978] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000008
[  270.991503] IP: [<ffffffffc050f6dd>] rapl_cpu_online+0x5d/0x69
[intel_rapl_perf]
[  270.992036] PGD 0
[  270.992047]
[  270.992366] Oops: 0002 [#1] SMP
[  270.992782] Modules linked in: qxl ttm drm_kms_helper drm kvm_intel
kvm irqbypass crct10dif_pclmul crc32_pclmul hid_generic joydev
ghash_clmulni_intel serio_raw pcspkr intel_rapl_perf ppdev evdev
parport_pc parport usbhid hid acpi_cpufreq button ip_tables x_tables
autofs4 ext4 crc16 jbd2 crc32c_generic fscrypto ecb mbcache
ata_generic virtio_net virtio_blk crc32c_intel aesni_intel aes_x86_64
glue_helper lrw gf128mul ablk_helper cryptd ata_piix libata uhci_hcd
ehci_hcd psmouse virtio_pci virtio_ring usbcore virtio i2c_piix4
scsi_mod usb_common floppy
[  270.995252] CPU: 10 PID: 628 Comm: cpuhp/10 Not tainted
4.9.0-3-amd64 #1 Debian 4.9.30-2+deb9u2
[  270.995650] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS ?-20160812_114322-buildbot 04/01/2014
[  270.996184] task: ffff9d5b73e6d140 task.stack: ffffb7f600f58000
[  270.996538] RIP: 0010:[<ffffffffc050f6dd>]  [<ffffffffc050f6dd>]
rapl_cpu_online+0x5d/0x69 [intel_rapl_perf]
[  270.997131] RSP: 0000:ffffb7f600f5bde8  EFLAGS: 00010206
[  270.997491] RAX: 0000000000000200 RBX: 000000000000000a RCX: 0000000000000000
[  270.998091] RDX: 0000000000000200 RSI: 0000000000000200 RDI: 0000000000000200
[  270.998459] RBP: 000000000000000a R08: fffffffffffffe00 R09: 0000000000000155
[  270.999064] R10: 00000000000001b6 R11: 000000000047f4d8 R12: 0000000000000000
[  270.999438] R13: ffffffffc050f680 R14: 0000000000000000 R15: 0000000000000000
[  271.000043] FS:  0000000000000000(0000) GS:ffff9d5b7a880000(0000)
knlGS:0000000000000000
[  271.000421] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  271.001052] CR2: 0000000000000008 CR3: 000000006fc07000 CR4: 00000000001406e0
[  271.001431] Stack:
[  271.002212]  000000000000000a 000000000000006a 0000000000000062
ffffffff95278008
[  271.002927]  ffff9d5b7a88dae0 ffffffff95e33ad0 000000017a618240
ffff9d5b73e6d140
[  271.003546]  ffff9d5b7a88dae0 000000000000000a 0000000000000062
0000000000000097
[  271.004393] Call Trace:
[  271.005200]  [<ffffffff95278008>] ? cpuhp_invoke_callback+0x78/0x3a0
[  271.006015]  [<ffffffff9527bb0a>] ? do_group_exit+0x3a/0xa0
[  271.006584]  [<ffffffff95278470>] ? cpuhp_up_callbacks+0x30/0xb0
[  271.007339]  [<ffffffff95299d30>] ? sort_range+0x20/0x20
[  271.008143]  [<ffffffff95278674>] ? cpuhp_thread_fun+0xf4/0x100
[  271.008937]  [<ffffffff95299e2e>] ? smpboot_thread_fn+0xfe/0x150
[  271.009742]  [<ffffffff952965d7>] ? kthread+0xd7/0xf0
[  271.010272]  [<ffffffff95296500>] ? kthread_park+0x60/0x60
[  271.011012]  [<ffffffff958064f5>] ? ret_from_fork+0x25/0x30
[  271.011496] Code: 24 00 00 4c 8b a4 d0 10 01 00 00 48 c7 c2 80 a0
00 00 48 01 ca e8 44 8b 01 d5 3b 05 92 40 a0 d5 7c 0e f0 48 0f ab 2d
c3 23 00 00 <41> 89 6c 24 08 5b 31 c0 5d 41 5c c3 0f 1f 44 00 00 53 31
db 48
[  271.012776] RIP  [<ffffffffc050f6dd>] rapl_cpu_online+0x5d/0x69
[intel_rapl_perf]
[  271.013301]  RSP <ffffb7f600f5bde8>
[  271.013794] CR2: 0000000000000008

similar bug was reported in :
https://lkml.org/lkml/2016/10/20/745
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=851577

On Tue, Aug 22, 2017 at 4:52 PM, Jack Wang <jinpu.wang@profitbricks.com> wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
>
> commit dd86e373e09fb16b83e8adf5c48c421a4ca76468 upstream
>
> The package management code in RAPL relies on package mapping being
> available before a CPU is started. This changed with:
>
>   9d85eb9119f4 ("x86/smpboot: Make logical package management more robust")
>
> because the ACPI/BIOS information turned out to be unreliable, but that
> left RAPL in broken state. This was not noticed because on a regular boot
> all CPUs are online before RAPL is initialized.
>
> A possible fix would be to reintroduce the mess which allocates a package
> data structure in CPU prepare and when it turns out to already exist in
> starting throw it away later in the CPU online callback. But that's a
> horrible hack and not required at all because RAPL becomes functional for
> perf only in the CPU online callback. That's correct because user space is
> not yet informed about the CPU being onlined, so nothing caan rely on RAPL
> being available on that particular CPU.
>
> Move the allocation to the CPU online callback and simplify the hotplug
> handling. At this point the package mapping is established and correct.
>
> This also adds a missing check for available package data in the
> event_init() function.
>
> Reported-by: Yasuaki Ishimatsu <yasu.isimatu@gmail.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Sebastian Siewior <bigeasy@linutronix.de>
> Cc: Stephane Eranian <eranian@google.com>
> Cc: Vince Weaver <vincent.weaver@maine.edu>
> Fixes: 9d85eb9119f4 ("x86/smpboot: Make logical package management more robust")
> Link: http://lkml.kernel.org/r/20170131230141.212593966@linutronix.de
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> [ jwang: backport to 4.9 fix Null pointer deref during hotplug cpu.]
> Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
> ---
>  arch/x86/events/intel/rapl.c | 58 +++++++++++++++++++-------------------------
>  include/linux/cpuhotplug.h   |  1 -
>  2 files changed, 25 insertions(+), 34 deletions(-)
>
> diff --git a/arch/x86/events/intel/rapl.c b/arch/x86/events/intel/rapl.c
> index 970c1de..4c1b7ea 100644
> --- a/arch/x86/events/intel/rapl.c
> +++ b/arch/x86/events/intel/rapl.c
> @@ -161,7 +161,13 @@ static u64 rapl_timer_ms;
>
>  static inline struct rapl_pmu *cpu_to_rapl_pmu(unsigned int cpu)
>  {
> -       return rapl_pmus->pmus[topology_logical_package_id(cpu)];
> +       unsigned int pkgid = topology_logical_package_id(cpu);
> +
> +       /*
> +        * The unsigned check also catches the '-1' return value for non
> +        * existent mappings in the topology map.
> +        */
> +       return pkgid < rapl_pmus->maxpkg ? rapl_pmus->pmus[pkgid] : NULL;
>  }
>
>  static inline u64 rapl_read_counter(struct perf_event *event)
> @@ -402,6 +408,8 @@ static int rapl_pmu_event_init(struct perf_event *event)
>
>         /* must be done before validate_group */
>         pmu = cpu_to_rapl_pmu(event->cpu);
> +       if (!pmu)
> +               return -EINVAL;
>         event->cpu = pmu->cpu;
>         event->pmu_private = pmu;
>         event->hw.event_base = msr;
> @@ -585,6 +593,19 @@ static int rapl_cpu_online(unsigned int cpu)
>         struct rapl_pmu *pmu = cpu_to_rapl_pmu(cpu);
>         int target;
>
> +       if (!pmu) {
> +               pmu = kzalloc_node(sizeof(*pmu), GFP_KERNEL, cpu_to_node(cpu));
> +               if (!pmu)
> +                       return -ENOMEM;
> +
> +               raw_spin_lock_init(&pmu->lock);
> +               INIT_LIST_HEAD(&pmu->active_list);
> +               pmu->pmu = &rapl_pmus->pmu;
> +               pmu->timer_interval = ms_to_ktime(rapl_timer_ms);
> +               rapl_hrtimer_init(pmu);
> +
> +               rapl_pmus->pmus[topology_logical_package_id(cpu)] = pmu;
> +       }
>         /*
>          * Check if there is an online cpu in the package which collects rapl
>          * events already.
> @@ -598,27 +619,6 @@ static int rapl_cpu_online(unsigned int cpu)
>         return 0;
>  }
>
> -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;
> -
> -       raw_spin_lock_init(&pmu->lock);
> -       INIT_LIST_HEAD(&pmu->active_list);
> -       pmu->pmu = &rapl_pmus->pmu;
> -       pmu->timer_interval = ms_to_ktime(rapl_timer_ms);
> -       pmu->cpu = -1;
> -       rapl_hrtimer_init(pmu);
> -       rapl_pmus->pmus[topology_logical_package_id(cpu)] = pmu;
> -       return 0;
> -}
> -
>  static int rapl_check_hw_unit(bool apply_quirk)
>  {
>         u64 msr_rapl_power_unit_bits;
> @@ -804,28 +804,21 @@ static int __init rapl_pmu_init(void)
>          * 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);
>         if (ret)
> -               goto out1;
> +               goto out;
>
>         ret = perf_pmu_register(&rapl_pmus->pmu, "power", -1);
>         if (ret)
> -               goto out2;
> +               goto out1;
>
>         rapl_advertise();
>         return 0;
>
> -out2:
> -       cpuhp_remove_state(CPUHP_AP_PERF_X86_RAPL_ONLINE);
>  out1:
> -       cpuhp_remove_state(CPUHP_PERF_X86_RAPL_PREP);
> +       cpuhp_remove_state(CPUHP_AP_PERF_X86_RAPL_ONLINE);
>  out:
>         pr_warn("Initialization failed (%d), disabled\n", ret);
>         cleanup_rapl_pmus();
> @@ -836,7 +829,6 @@ module_init(rapl_pmu_init);
>  static void __exit intel_rapl_exit(void)
>  {
>         cpuhp_remove_state_nocalls(CPUHP_AP_PERF_X86_RAPL_ONLINE);
> -       cpuhp_remove_state_nocalls(CPUHP_PERF_X86_RAPL_PREP);
>         perf_pmu_unregister(&rapl_pmus->pmu);
>         cleanup_rapl_pmus();
>  }
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index ba1cad7..965cc56 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -10,7 +10,6 @@ enum cpuhp_state {
>         CPUHP_PERF_X86_PREPARE,
>         CPUHP_PERF_X86_UNCORE_PREP,
>         CPUHP_PERF_X86_AMD_UNCORE_PREP,
> -       CPUHP_PERF_X86_RAPL_PREP,
>         CPUHP_PERF_BFIN,
>         CPUHP_PERF_POWER,
>         CPUHP_PERF_SUPERH,
> --
> 2.7.4
>



-- 
Jack Wang
Linux Kernel Developer

ProfitBricks GmbH
Greifswalder Str. 207
D - 10405 Berlin

Tel:       +49 30 577 008  042
Fax:      +49 30 577 008 299
Email:    jinpu.wang@profitbricks.com
URL:      https://www.profitbricks.de

Sitz der Gesellschaft: Berlin
Registergericht: Amtsgericht Charlottenburg, HRB 125506 B
Geschäftsführer: Achim Weiss

Re: [PATCH stable 4.9] perf/x86/intel/rapl: Make package handling more robust

[Greg Kroah-Hartman]

On Tue, Aug 22, 2017 at 04:59:14PM +0200, Jinpu Wang wrote:
> Sorry, this is a backport for stable 4.9, Greg please consider to
> apply to 4.9, this fix NULL pointer deref during cpu hot-add in debian
> 9 VM.

Can you resend it in a format that I can apply it in?

thanks,

greg k-h

Re: [PATCH stable 4.9] perf/x86/intel/rapl: Make package handling more robust

[Jinpu Wang]

[-- Attachment #1: Type: text/plain, Size: 955 bytes --]

On Tue, Aug 22, 2017 at 7:54 PM, Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Tue, Aug 22, 2017 at 04:59:14PM +0200, Jinpu Wang wrote:
>> Sorry, this is a backport for stable 4.9, Greg please consider to
>> apply to 4.9, this fix NULL pointer deref during cpu hot-add in debian
>> 9 VM.
>
> Can you resend it in a format that I can apply it in?
>
> thanks,
>
> greg k-h

Thanks Greg for reply.

Please see file attached, I hope this is ok for you. if not, I can
send a V2 version.
I will make sure next time send the patch properly via git-send-email.

-- 
Jack Wang
Linux Kernel Developer

ProfitBricks GmbH
Greifswalder Str. 207
D - 10405 Berlin

Tel:       +49 30 577 008  042
Fax:      +49 30 577 008 299
Email:    jinpu.wang@profitbricks.com
URL:      https://www.profitbricks.de

Sitz der Gesellschaft: Berlin
Registergericht: Amtsgericht Charlottenburg, HRB 125506 B
Geschäftsführer: Achim Weiss

[-- Attachment #2: 0001-perf-x86-intel-rapl-Make-package-handling-more-robus.patch --]
[-- Type: text/x-patch, Size: 6045 bytes --]

From 1d7c1e12da6b02cdba614958b5f8ab82337bb197 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 31 Jan 2017 23:58:38 +0100
Subject: [PATCH] perf/x86/intel/rapl: Make package handling more robust

commit dd86e373e09fb16b83e8adf5c48c421a4ca76468 upstream

The package management code in RAPL relies on package mapping being
available before a CPU is started. This changed with:

  9d85eb9119f4 ("x86/smpboot: Make logical package management more robust")

because the ACPI/BIOS information turned out to be unreliable, but that
left RAPL in broken state. This was not noticed because on a regular boot
all CPUs are online before RAPL is initialized.

A possible fix would be to reintroduce the mess which allocates a package
data structure in CPU prepare and when it turns out to already exist in
starting throw it away later in the CPU online callback. But that's a
horrible hack and not required at all because RAPL becomes functional for
perf only in the CPU online callback. That's correct because user space is
not yet informed about the CPU being onlined, so nothing caan rely on RAPL
being available on that particular CPU.

Move the allocation to the CPU online callback and simplify the hotplug
handling. At this point the package mapping is established and correct.

This also adds a missing check for available package data in the
event_init() function.

Reported-by: Yasuaki Ishimatsu <yasu.isimatu@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Stephane Eranian <eranian@google.com>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Fixes: 9d85eb9119f4 ("x86/smpboot: Make logical package management more robust")
Link: http://lkml.kernel.org/r/20170131230141.212593966@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
[ jwang: backport to 4.9 fix Null pointer deref during hotplug cpu.]
Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
---
 arch/x86/events/intel/rapl.c | 58 +++++++++++++++++++-------------------------
 include/linux/cpuhotplug.h   |  1 -
 2 files changed, 25 insertions(+), 34 deletions(-)

diff --git a/arch/x86/events/intel/rapl.c b/arch/x86/events/intel/rapl.c
index 970c1de..4c1b7ea 100644
--- a/arch/x86/events/intel/rapl.c
+++ b/arch/x86/events/intel/rapl.c
@@ -161,7 +161,13 @@ static u64 rapl_timer_ms;
 
 static inline struct rapl_pmu *cpu_to_rapl_pmu(unsigned int cpu)
 {
-	return rapl_pmus->pmus[topology_logical_package_id(cpu)];
+	unsigned int pkgid = topology_logical_package_id(cpu);
+
+	/*
+	 * The unsigned check also catches the '-1' return value for non
+	 * existent mappings in the topology map.
+	 */
+	return pkgid < rapl_pmus->maxpkg ? rapl_pmus->pmus[pkgid] : NULL;
 }
 
 static inline u64 rapl_read_counter(struct perf_event *event)
@@ -402,6 +408,8 @@ static int rapl_pmu_event_init(struct perf_event *event)
 
 	/* must be done before validate_group */
 	pmu = cpu_to_rapl_pmu(event->cpu);
+	if (!pmu)
+		return -EINVAL;
 	event->cpu = pmu->cpu;
 	event->pmu_private = pmu;
 	event->hw.event_base = msr;
@@ -585,6 +593,19 @@ static int rapl_cpu_online(unsigned int cpu)
 	struct rapl_pmu *pmu = cpu_to_rapl_pmu(cpu);
 	int target;
 
+	if (!pmu) {
+		pmu = kzalloc_node(sizeof(*pmu), GFP_KERNEL, cpu_to_node(cpu));
+		if (!pmu)
+			return -ENOMEM;
+
+		raw_spin_lock_init(&pmu->lock);
+		INIT_LIST_HEAD(&pmu->active_list);
+		pmu->pmu = &rapl_pmus->pmu;
+		pmu->timer_interval = ms_to_ktime(rapl_timer_ms);
+		rapl_hrtimer_init(pmu);
+
+		rapl_pmus->pmus[topology_logical_package_id(cpu)] = pmu;
+	}
 	/*
 	 * Check if there is an online cpu in the package which collects rapl
 	 * events already.
@@ -598,27 +619,6 @@ static int rapl_cpu_online(unsigned int cpu)
 	return 0;
 }
 
-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;
-
-	raw_spin_lock_init(&pmu->lock);
-	INIT_LIST_HEAD(&pmu->active_list);
-	pmu->pmu = &rapl_pmus->pmu;
-	pmu->timer_interval = ms_to_ktime(rapl_timer_ms);
-	pmu->cpu = -1;
-	rapl_hrtimer_init(pmu);
-	rapl_pmus->pmus[topology_logical_package_id(cpu)] = pmu;
-	return 0;
-}
-
 static int rapl_check_hw_unit(bool apply_quirk)
 {
 	u64 msr_rapl_power_unit_bits;
@@ -804,28 +804,21 @@ static int __init rapl_pmu_init(void)
 	 * 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);
 	if (ret)
-		goto out1;
+		goto out;
 
 	ret = perf_pmu_register(&rapl_pmus->pmu, "power", -1);
 	if (ret)
-		goto out2;
+		goto out1;
 
 	rapl_advertise();
 	return 0;
 
-out2:
-	cpuhp_remove_state(CPUHP_AP_PERF_X86_RAPL_ONLINE);
 out1:
-	cpuhp_remove_state(CPUHP_PERF_X86_RAPL_PREP);
+	cpuhp_remove_state(CPUHP_AP_PERF_X86_RAPL_ONLINE);
 out:
 	pr_warn("Initialization failed (%d), disabled\n", ret);
 	cleanup_rapl_pmus();
@@ -836,7 +829,6 @@ module_init(rapl_pmu_init);
 static void __exit intel_rapl_exit(void)
 {
 	cpuhp_remove_state_nocalls(CPUHP_AP_PERF_X86_RAPL_ONLINE);
-	cpuhp_remove_state_nocalls(CPUHP_PERF_X86_RAPL_PREP);
 	perf_pmu_unregister(&rapl_pmus->pmu);
 	cleanup_rapl_pmus();
 }
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index ba1cad7..965cc56 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -10,7 +10,6 @@ enum cpuhp_state {
 	CPUHP_PERF_X86_PREPARE,
 	CPUHP_PERF_X86_UNCORE_PREP,
 	CPUHP_PERF_X86_AMD_UNCORE_PREP,
-	CPUHP_PERF_X86_RAPL_PREP,
 	CPUHP_PERF_BFIN,
 	CPUHP_PERF_POWER,
 	CPUHP_PERF_SUPERH,
-- 
2.7.4

Re: [PATCH stable 4.9] perf/x86/intel/rapl: Make package handling more robust

[Greg Kroah-Hartman]

On Wed, Aug 23, 2017 at 10:09:39AM +0200, Jinpu Wang wrote:
> On Tue, Aug 22, 2017 at 7:54 PM, Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> > On Tue, Aug 22, 2017 at 04:59:14PM +0200, Jinpu Wang wrote:
> >> Sorry, this is a backport for stable 4.9, Greg please consider to
> >> apply to 4.9, this fix NULL pointer deref during cpu hot-add in debian
> >> 9 VM.
> >
> > Can you resend it in a format that I can apply it in?
> >
> > thanks,
> >
> > greg k-h
> 
> Thanks Greg for reply.
> 
> Please see file attached, I hope this is ok for you. if not, I can
> send a V2 version.
> I will make sure next time send the patch properly via git-send-email.

That worked, thanks.

greg k-h