Patch "powerpc/perf: Fix book3s kernel to userspace backtraces" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

powerpc/perf: Fix book3s kernel to userspace backtraces

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-perf-fix-book3s-kernel-to-userspace-backtraces.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Anton Blanchard <[email protected]>
Date: Tue, 26 May 2015 15:10:24 +1000
Subject: powerpc/perf: Fix book3s kernel to userspace backtraces

From: Anton Blanchard <[email protected]>

commit 72e349f1124a114435e599479c9b8d14bfd1ebcd upstream.

When we take a PMU exception or a software event we call perf_read_regs(). This overloads regs->result with a boolean that describes if we should use the sampled instruction address register (SIAR) or the regs.

If the exception is in kernel, we start with the kernel regs and backtrace through the kernel stack. At this point we switch to the userspace regs and backtrace the user stack with perf_callchain_user().

Unfortunately these regs have not got the perf_read_regs() treatment, so regs->result could be anything. If it is non zero, perf_instruction_pointer() decides to use the SIAR, and we get issues like this:

0.11% qemu-system-ppc [kernel.kallsyms] [k] _raw_spin_lock_irqsave
|
---_raw_spin_lock_irqsave
|
|--52.35%-- 0
| |
| |--46.39%-- __hrtimer_start_range_ns
| | kvmppc_run_core
| | kvmppc_vcpu_run_hv
| | kvmppc_vcpu_run
| | kvm_arch_vcpu_ioctl_run
| | kvm_vcpu_ioctl
| | do_vfs_ioctl
| | sys_ioctl
| | system_call
| | |
| | |--67.08%-- _raw_spin_lock_irqsave <--- hi mum
| | | |
| | | --100.00%-- 0x7e714
| | | 0x7e714

Notice the bogus _raw_spin_irqsave when we transition from kernel (system_call) to userspace (0x7e714). We inserted what was in the SIAR.

Add a check in regs_use_siar() to check that the regs in question are from a PMU exception. With this fix the backtrace makes sense:

0.47% qemu-system-ppc [kernel.vmlinux] [k] _raw_spin_lock_irqsave
|
---_raw_spin_lock_irqsave
|
|--53.83%-- 0
| |
| |--44.73%-- hrtimer_try_to_cancel
| | kvmppc_start_thread
| | kvmppc_run_core
| | kvmppc_vcpu_run_hv
| | kvmppc_vcpu_run
| | kvm_arch_vcpu_ioctl_run
| | kvm_vcpu_ioctl
| | do_vfs_ioctl
| | sys_ioctl
| | system_call
| | __ioctl
| | 0x7e714
| | 0x7e714

---
 arch/powerpc/perf/core-book3s.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

@@ -131,7 +131,16 @@ static void pmao_restore_workaround(bool
 
 static bool regs_use_siar(struct pt_regs *regs)
 {
-	return !!regs->result;
+	/*
+	 * When we take a performance monitor exception the regs are setup
+	 * using perf_read_regs() which overloads some fields, in particular
+	 * regs->result to tell us whether to use SIAR.
+	 *
+	 * However if the regs are from another exception, eg. a syscall, then
+	 * they have not been setup using perf_read_regs() and so regs->result
+	 * is something random.
+	 */
+	return ((TRAP(regs) == 0xf00) && regs->result);
 }
 
 /*

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/powerpc-perf-fix-book3s-kernel-to-userspace-backtraces.patch