[PATCH 4.9 079/240] tracing: Restructure trace_clock_global() to never block

Greg Kroah-Hartman
2021-05-20T11:09:06Z

From: Steven Rostedt (VMware) <rostedt@goodmis.org>

commit aafe104aa9096827a429bc1358f8260ee565b7cc upstream.

It was reported that a fix to the ring buffer recursion detection would
cause a hung machine when performing suspend / resume testing. The
following backtrace was extracted from debugging that case:

Call Trace:
trace_clock_global+0x91/0xa0
__rb_reserve_next+0x237/0x460
ring_buffer_lock_reserve+0x12a/0x3f0
trace_buffer_lock_reserve+0x10/0x50
__trace_graph_return+0x1f/0x80
trace_graph_return+0xb7/0xf0
? trace_clock_global+0x91/0xa0
ftrace_return_to_handler+0x8b/0xf0
? pv_hash+0xa0/0xa0
return_to_handler+0x15/0x30
? ftrace_graph_caller+0xa0/0xa0
? trace_clock_global+0x91/0xa0
? __rb_reserve_next+0x237/0x460
? ring_buffer_lock_reserve+0x12a/0x3f0
? trace_event_buffer_lock_reserve+0x3c/0x120
? trace_event_buffer_reserve+0x6b/0xc0
? trace_event_raw_event_device_pm_callback_start+0x125/0x2d0
? dpm_run_callback+0x3b/0xc0
? pm_ops_is_empty+0x50/0x50
? platform_get_irq_byname_optional+0x90/0x90
? trace_device_pm_callback_start+0x82/0xd0
? dpm_run_callback+0x49/0xc0

With the following RIP:

RIP: 0010:native_queued_spin_lock_slowpath+0x69/0x200

Since the fix to the recursion detection would allow a single recursion to
happen while tracing, this lead to the trace_clock_global() taking a spin
lock and then trying to take it again:

ring_buffer_lock_reserve() {
trace_clock_global() {
arch_spin_lock() {
queued_spin_lock_slowpath() {
/* lock taken */
(something else gets traced by function graph tracer)
ring_buffer_lock_reserve() {
trace_clock_global() {
arch_spin_lock() {
queued_spin_lock_slowpath() {
/* DEAD LOCK! */

Tracing should *never* block, as it can lead to strange lockups like the
above.

Restructure the trace_clock_global() code to instead of simply taking a
lock to update the recorded "prev_time" simply use it, as two events
happening on two different CPUs that calls this at the same time, really
doesn't matter which one goes first. Use a trylock to grab the lock for
updating the prev_time, and if it fails, simply try again the next time.
If it failed to be taken, that means something else is already updating
it.

Link: https://lkml.kernel.org/r/20210430121758.650b6e8a@gandalf.local.home

Cc: stable@vger.kernel.org
Tested-by: Konstantin Kharlamov <hi-angel@yandex.ru>
Tested-by: Todd Brandt <todd.e.brandt@linux.intel.com>
Fixes: b02414c8f045 ("ring-buffer: Fix recursion protection transitions between interrupt context") # started showing the problem
Fixes: 14131f2f98ac3 ("tracing: implement trace_clock_*() APIs") # where the bug happened
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=212761
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/trace/trace_clock.c | 48 ++++++++++++++++++++++++++++++---------------
1 file changed, 32 insertions(+), 16 deletions(-)

--- a/kernel/trace/trace_clock.c
+++ b/kernel/trace/trace_clock.c
@@ -93,33 +93,49 @@ u64 notrace trace_clock_global(void)
{
unsigned long flags;
int this_cpu;
- u64 now;
+ u64 now, prev_time;

local_irq_save(flags);

this_cpu = raw_smp_processor_id();
- now = sched_clock_cpu(this_cpu);
+
/*
- * If in an NMI context then dont risk lockups and return the
- * cpu_clock() time:
+ * The global clock "guarantees" that the events are ordered
+ * between CPUs. But if two events on two different CPUS call
+ * trace_clock_global at roughly the same time, it really does
+ * not matter which one gets the earlier time. Just make sure
+ * that the same CPU will always show a monotonic clock.
+ *
+ * Use a read memory barrier to get the latest written
+ * time that was recorded.
*/
- if (unlikely(in_nmi()))
- goto out;
+ smp_rmb();
+ prev_time = READ_ONCE(trace_clock_struct.prev_time);
+ now = sched_clock_cpu(this_cpu);

- arch_spin_lock(&trace_clock_struct.lock);
+ /* Make sure that now is always greater than prev_time */
+ if ((s64)(now - prev_time) < 0)
+ now = prev_time + 1;

/*
- * TODO: if this happens often then maybe we should reset
- * my_scd->clock to prev_time+1, to make sure
- * we start ticking with the local clock from now on?
+ * If in an NMI context then dont risk lockups and simply return
+ * the current time.
*/
- if ((s64)(now - trace_clock_struct.prev_time) < 0)
- now = trace_clock_struct.prev_time + 1;
-
- trace_clock_struct.prev_time = now;
-
- arch_spin_unlock(&trace_clock_struct.lock);
+ if (unlikely(in_nmi()))
+ goto out;

+ /* Tracing can cause strange recursion, always use a try lock */
+ if (arch_spin_trylock(&trace_clock_struct.lock)) {
+ /* Reread prev_time in case it was already updated */
+ prev_time = READ_ONCE(trace_clock_struct.prev_time);
+ if ((s64)(now - prev_time) < 0)
+ now = prev_time + 1;
+
+ trace_clock_struct.prev_time = now;
+
+ /* The unlock acts as the wmb for the above rmb */
+ arch_spin_unlock(&trace_clock_struct.lock);
+ }
out:
local_irq_restore(flags);