wwbmmm commented on code in PR #2225:
URL: https://github.com/apache/brpc/pull/2225#discussion_r1200411426
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src/butil/containers/doubly_buffered_data.h:
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@@ -49,10 +51,38 @@ namespace butil {
// foreground and background, lock thread-local mutexes one by one to make
// sure all existing Read() finish and later Read() see new foreground,
// then modify background(foreground before flip) again.
+//
+// But, when `AllowSuspendBthread=false', it is not allowed to suspend bthread
+// while reading. Otherwise, it may cause deadlock.
+//
+//
+// --- `AllowSuspendBthread=true' ---
+// It is allowed to suspend bthread while reading.
+// It is not allowed to use non-Void TLS.
+// If bthread will not be suspended while reading, it also makes Read() almost
+// lock-free by making Modify() *much* slower.
+// If bthread will be suspended while reading, there is competition among
+// bthreads using the same Wrapper.
+//
+// Read(): Begin with thread-local reference count of foreground instance
+// incremented by one which be protected by a thread-local mutex, then read
+// the foreground instance which will not be changed before its all
thread-local
+// reference count become zero. At last, after the query completes,
thread-local
+// reference count of foreground instance will be decremented by one, and if
+// it becomes zero, notifies Modify().
+//
+// Modify(): Modify background instance which is not used by any Read(), flip
+// foreground and background, lock thread-local mutexes one by one and wait
+// until thread-local reference counts which be protected by a thread-local
+// mutex become 0 to make sure all existing Read() finish and later Read()
+// see new foreground, then modify background(foreground before flip) again.
class Void { };
-template <typename T, typename TLS = Void>
+template <typename T> struct is_Void : false_type { };
Review Comment:
IsVoid 可以么
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