This is a note to let you know that I've just added the patch titled tracing: Restructure trace_clock_global() to never block to the 4.4-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: tracing-restructure-trace_clock_global-to-never-block.patch and it can be found in the queue-4.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From aafe104aa9096827a429bc1358f8260ee565b7cc Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx> Date: Fri, 30 Apr 2021 12:17:58 -0400 Subject: tracing: Restructure trace_clock_global() to never block From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx Cc: stable@xxxxxxxxxxxxxxx Tested-by: Konstantin Kharlamov <hi-angel@xxxxxxxxx> Tested-by: Todd Brandt <todd.e.brandt@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- 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); Patches currently in stable-queue which might be from rostedt@xxxxxxxxxxx are queue-4.4/tracing-restructure-trace_clock_global-to-never-block.patch queue-4.4/tracing-use-strlcpy-instead-of-strcpy-in-__trace_find_cmdline.patch queue-4.4/tracing-map-all-pids-to-command-lines.patch queue-4.4/tracing-treat-recording-comm-for-idle-task-as-a-success.patch queue-4.4/ftrace-handle-commands-when-closing-set_ftrace_filter-file.patch