[PATCH V2] powerpc/perf: Fix IMC initialization crash

[Anju T Sudhakar <anju@linux.vnet.ibm.com>]

Call trace observed with latest firmware, and upstream kernel.

[   14.499938] NIP [c0000000000f318c] init_imc_pmu+0x8c/0xcf0
[   14.499973] LR [c0000000000f33f8] init_imc_pmu+0x2f8/0xcf0
[   14.500007] Call Trace:
[   14.500027] [c000003fed18f710] [c0000000000f33c8] init_imc_pmu+0x2c8/0xcf0 (unreliable)
[   14.500080] [c000003fed18f800] [c0000000000b5ec0] opal_imc_counters_probe+0x300/0x400
[   14.500132] [c000003fed18f900] [c000000000807ef4] platform_drv_probe+0x64/0x110
[   14.500185] [c000003fed18f980] [c000000000804b58] driver_probe_device+0x3d8/0x580
[   14.500236] [c000003fed18fa10] [c000000000804e4c] __driver_attach+0x14c/0x1a0
[   14.500302] [c000003fed18fa90] [c00000000080156c] bus_for_each_dev+0x8c/0xf0
[   14.500353] [c000003fed18fae0] [c000000000803fa4] driver_attach+0x34/0x50
[   14.500397] [c000003fed18fb00] [c000000000803688] bus_add_driver+0x298/0x350
[   14.500449] [c000003fed18fb90] [c00000000080605c] driver_register+0x9c/0x180
[   14.500500] [c000003fed18fc00] [c000000000807dec] __platform_driver_register+0x5c/0x70
[   14.500552] [c000003fed18fc20] [c00000000101cee0] opal_imc_driver_init+0x2c/0x40
[   14.500603] [c000003fed18fc40] [c00000000000d084] do_one_initcall+0x64/0x1d0
[   14.500654] [c000003fed18fd00] [c00000000100434c] kernel_init_freeable+0x280/0x374
[   14.500705] [c000003fed18fdc0] [c00000000000d314] kernel_init+0x24/0x160
[   14.500750] [c000003fed18fe30] [c00000000000b4e8] ret_from_kernel_thread+0x5c/0x74
[   14.500799] Instruction dump:
[   14.500827] 4082024c 2f890002 419e054c 2e890003 41960094 2e890001 3ba0ffea 419602d8 
[   14.500884] 419e0290 2f890003 419e02a8 e93e0118 <e8690018> 2fa30000 419e0010 4827ba41 
[   14.500945] ---[ end trace 27b734ad26f1add4 ]---
[   15.908719] 
[   16.908869] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000007
[   16.908869] 
[   18.125813] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000007]

While registering nest imc at init, cpu-hotplug callback `nest_pmu_cpumask_init()`
makes an opal call to stop the engine. And if the OPAL call fails, 
imc_common_cpuhp_mem_free() is invoked to cleanup memory and cpuhotplug setup.

But when cleaning up the attribute group, we were dereferencing the attribute
element array without checking whether the backing element is not NULL. This
causes the kernel panic.

Add a check for the backing element prior to dereferencing the attribute element,
to handle the failing case gracefully.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Reported-by: Pridhiviraj Paidipeddi <ppaidipe@linux.vnet.ibm.com>
---
 arch/powerpc/perf/imc-pmu.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/perf/imc-pmu.c b/arch/powerpc/perf/imc-pmu.c
index 9ccac86f3463..001504b0e800 100644
--- a/arch/powerpc/perf/imc-pmu.c
+++ b/arch/powerpc/perf/imc-pmu.c
@@ -1148,7 +1148,8 @@ static void imc_common_cpuhp_mem_free(struct imc_pmu *pmu_ptr)
 	}
 
 	/* Only free the attr_groups which are dynamically allocated  */
-	kfree(pmu_ptr->attr_groups[IMC_EVENT_ATTR]->attrs);
+	if (pmu_ptr->attr_groups[IMC_EVENT_ATTR])
+		kfree(pmu_ptr->attr_groups[IMC_EVENT_ATTR]->attrs);
 	kfree(pmu_ptr->attr_groups[IMC_EVENT_ATTR]);
 	kfree(pmu_ptr);
 	return;
-- 
2.14.1