[PATCH 3.16 022/204] powerpc/sysrq: Fix oops whem ppmu is not registered

[Ben Hutchings]

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx>

commit 4917fcb58cc73f6b81455e3c5f960144809ddf1a upstream.

Kernel crashes if power pmu is not registered and user tries to dump
regs with 'echo p > /proc/sysrq-trigger'. Sample log:

  Unable to handle kernel paging request for data at address 0x00000008
  Faulting instruction address: 0xc0000000000d52f0

  NIP [c0000000000d52f0] perf_event_print_debug+0x10/0x230
  LR [c00000000058a938] sysrq_handle_showregs+0x38/0x50
  Call Trace:
   printk+0x38/0x4c (unreliable)
   __handle_sysrq+0xe4/0x270
   write_sysrq_trigger+0x64/0x80
   proc_reg_write+0x80/0xd0
   __vfs_write+0x40/0x200
   vfs_write+0xc8/0x240
   SyS_write+0x60/0x110
   system_call+0x58/0x6c

Fixes: 5f6d0380c640 ("powerpc/perf: Define perf_event_print_debug() to print PMU register values")
Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Kamalesh Babulal <kamalesh@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/powerpc/perf/core-book3s.c | 5 +++++
 1 file changed, 5 insertions(+)

--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -755,6 +755,11 @@ void perf_event_print_debug(void)
 	u32 pmcs[MAX_HWEVENTS];
 	int i;
 
+	if (!ppmu) {
+		pr_info("Performance monitor hardware not registered.\n");
+		return;
+	}
+
 	if (!ppmu->n_counter)
 		return;