tracing: Restructure trace_clock_global() to never block

In the Linux kernel, the following vulnerability has been resolved:

tracing: Restructure trace_clock_global() to never block

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.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=212761