trace: allow atomic changes of _log_backtrace via tracepoint_base::log_backtrace()

Sampler will need to set and later restore value of this option.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

core/trace.cc

@@ -291,11 +291,11 @@ std::unordered_set<tracepoint_id>& tracepoint_base::known_ids()
     return _known_ids;
 }
 
-bool tracepoint_base::_log_backtrace;
+std::atomic<bool> tracepoint_base::_log_backtrace;
 
-void tracepoint_base::log_backtraces()
+bool tracepoint_base::log_backtraces(bool should_log)
 {
-    _log_backtrace = true;
+    return _log_backtrace.exchange(should_log);
 }
 
 void tracepoint_base::do_log_backtrace(trace_record* tr, u8*& buffer)

include/osv/trace.hh

@@ -273,7 +273,7 @@ public:
                              const char* _name, const char* _format);
     ~tracepoint_base();
     void enable();
-    static void log_backtraces();
+    static bool log_backtraces(bool should_log = true);
     void add_probe(probe* p);
     void del_probe(probe* p);
     tracepoint_id id;
@@ -311,7 +311,7 @@ private:
     void deactivate(const tracepoint_id &, void * site, void * slow_path);
     void update();
     static std::unordered_set<tracepoint_id>& known_ids();
-    static bool _log_backtrace;
+    static std::atomic<bool> _log_backtrace;
     static const size_t backtrace_len = 10;
 };