msharee9 commented on a change in pull request #743: Minificpp 1169 - Simplify
C2 metrics collection and reporting
URL: https://github.com/apache/nifi-minifi-cpp/pull/743#discussion_r401721628
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File path: libminifi/src/utils/BackTrace.cpp
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@@ -80,25 +80,21 @@ BackTrace TraceResolver::getBackTrace(std::string
thread_name, std::thread::nati
// lock so that we only perform one backtrace at a time.
#ifdef HAS_EXECINFO
std::lock_guard<std::mutex> lock(mutex_);
-
- caller_handle_ = pthread_self();
- thread_handle_ = thread_handle;
trace_ = BackTrace(std::move(thread_name));
- if (0 == thread_handle_ || pthread_equal(caller_handle_, thread_handle)) {
+ if (0 == thread_handle || pthread_equal(pthread_self(), thread_handle)) {
pull_trace();
} else {
- if (thread_handle_ == 0) {
+ if (thread_handle == 0) {
return std::move(trace_);
}
emplace_handler();
- if (pthread_kill(thread_handle_, SIGUSR2) != 0) {
+ std::unique_lock<std::mutex> ulock(trace_mutex_);
+ if (pthread_kill(thread_handle, SIGUSR2) != 0) {
return std::move(trace_);
}
- sigset_t mask;
- sigfillset(&mask);
- sigdelset(&mask, SIGUSR2);
- sigsuspend(&mask);
Review comment:
There was a race condition here which lead to a deadlock.
Caller thread (Thread that is interested in pulling backtrace of other
thread) acquired lock on a mutex (worker_queue_mutex_), and sent a SIGUSR2
signal to another thread (thread_handle_) but before the caller thread goes
into a suspended state or may be even before it removes the SIGUSR2 from its
blocking signal set (sigdelset) , the callee while executing its thread
handler, sent a SIGUSR2 signal back to the caller and at this time the caller
thread will run through its signal handler and after returning from signal
handler it goes back to suspended state. From this state there is no way of
waking it up.
The callee was waiting on a condition variable tied to the mutex caller was
holding thus creating a deadlock.
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