[PATCH] powerpc/stacktrace: fix raise_backtrace_ipi() logic

[Nathan Lynch <nathanl@linux.ibm.com>]

When smp_send_safe_nmi_ipi() indicates that the target CPU has
responded to the IPI, skip the remote paca inspection
fallback. Otherwise both the sending and target CPUs attempt the
backtrace, usually creating a misleading ("didn't respond to backtrace
IPI" is wrong) and interleaved mess:

[ 1658.929157][    C7] rcu: Stack dump where RCU GP kthread last ran:
[ 1658.929223][    C7] Sending NMI from CPU 7 to CPUs 1:
[ 1658.929303][    C1] NMI backtrace for cpu 1
[ 1658.929303][    C7] CPU 1 didn't respond to backtrace IPI, inspecting paca.
[ 1658.929362][    C1] CPU: 1 PID: 325 Comm: kworker/1:1H Tainted: G        W   E     5.13.0-rc2+ #46
[ 1658.929405][    C7] irq_soft_mask: 0x01 in_mce: 0 in_nmi: 0 current: 325 (kworker/1:1H)
[ 1658.929465][    C1] Workqueue: events_highpri test_work_fn [test_lockup]
[ 1658.929549][    C7] Back trace of paca->saved_r1 (0xc0000000057fb400) (possibly stale):
[ 1658.929592][    C1] NIP:  c00000000002cf50 LR: c008000000820178 CTR: c00000000002cfa0

Verified using the test_lockup module, e.g.

$ echo 5 > /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout
$ insmod test_lockup.ko time_secs=1 iterations=10 state=R lock_rcu \
  touch_softlockup all_cpus

Fixes: 5cc05910f26e ("powerpc/64s: Wire up arch_trigger_cpumask_backtrace()")
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
---
 arch/powerpc/kernel/stacktrace.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
index 1deb1bf331dd..e0ccc5a46d7e 100644
--- a/arch/powerpc/kernel/stacktrace.c
+++ b/arch/powerpc/kernel/stacktrace.c
@@ -174,11 +174,14 @@ static void raise_backtrace_ipi(cpumask_t *mask)
 {
 	unsigned int cpu;
 
+	if (cpumask_test_cpu(smp_processor_id(), mask)) {
+		handle_backtrace_ipi(NULL);
+		cpumask_clear_cpu(smp_processor_id(), mask);
+	}
+
 	for_each_cpu(cpu, mask) {
-		if (cpu == smp_processor_id())
-			handle_backtrace_ipi(NULL);
-		else
-			smp_send_safe_nmi_ipi(cpu, handle_backtrace_ipi, 5 * USEC_PER_SEC);
+		if (smp_send_safe_nmi_ipi(cpu, handle_backtrace_ipi, 5 * USEC_PER_SEC))
+			cpumask_clear_cpu(cpu, mask);
 	}
 
 	for_each_cpu(cpu, mask) {
-- 
2.31.1