The default TracingController (used by d8 and Node) has some concurrency
issues. The new test flushes these out, when a second thread logs trace
events while the main thread calls StopTracing().
- Use an acquire load in UpdateCategoryGroupEnabledFlags() because this
was racing with GetCategoryGroupEnabled() where a new category is
added in the slow path. g_category_groups is append-only, but
reads/writes to g_category_index need to be correctly ordered so that
new categories are added and only then is the change to the index
visible. The relaxed load ignored this and caused unsynchronized
read/write.
- Use a relaxed load in ~ScopedTracer() to access category_group_enabled
as this previously used a non-atomic operation which caused a race
with UpdateCategoryGroupEnabledFlag() which does a relaxed store.
- Replace TracingController::mode_ with an atomic bool as read/writes to
mode_ were not synchronized and caused TSAN errors. It only has two
states and it doesn't seem like we will extend this so just convert it
to bool.
- Take the lock around calling trace_object->Initialize in
AddTraceEvent(), and around trace_buffer_->Flush() in StopTracing().
These two raced previously as the underlying TraceBufferRingBuffer
passes out pointers to TraceObjects in a synchronized way, but the
caller (AddTraceEvent) then writes into the object without
synchronization. This leads to races when Flush() is called, at which
time TraceBufferRingBuffer assumes that all the pointers it handed out
are to valid, initialized TraceObjects - which is not true because
AddTraceEvent may still be calling Initialize on them. This could be
the cause of issues in Node.js where the last line of tracing/logging
sometimes gets cut off. This is kind of a band-aid solution - access
to the TraceObjects handed out by the ring buffer really needs proper
synchronization which at this point would require redesign. It's quite
likely we will replace this with Perfetto in the near future so not
much point investing in this code right now.
- Enable TracingCpuProfiler test which was flaky due to these bugs.
Bug: v8:8821
Change-Id: I141296800c6906ac0e7f3f21dd16d861b07dae62
Reviewed-on: https://chromium-review.googlesource.com/c/1477283
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#59752}
51e80efd12