Add TODO in fd_global_shutdown()

src/core/lib/iomgr/ev_epoll1_linux.cc

@@ -273,6 +273,10 @@ static gpr_mu fork_fd_list_mu;
 static void fd_global_init(void) { gpr_mu_init(&fd_freelist_mu); }
 
 static void fd_global_shutdown(void) {
+  // TODO(guantaol): We don't have a reasonable explanation about this
+  // lock()/unlock() pattern. It can be a valid barrier if there is at most one
+  // pending lock() at this point. Otherwise, there is still a possibility of
+  // use-after-free race. Need to reason about the code and/or clean it up.
   gpr_mu_lock(&fd_freelist_mu);
   gpr_mu_unlock(&fd_freelist_mu);
   while (fd_freelist != nullptr) {

src/core/lib/iomgr/ev_epollex_linux.cc

@@ -403,6 +403,10 @@ static void unref_by(grpc_fd* fd, int n) {
 static void fd_global_init(void) { gpr_mu_init(&fd_freelist_mu); }
 
 static void fd_global_shutdown(void) {
+  // TODO(guantaol): We don't have a reasonable explanation about this
+  // lock()/unlock() pattern. It can be a valid barrier if there is at most one
+  // pending lock() at this point. Otherwise, there is still a possibility of
+  // use-after-free race. Need to reason about the code and/or clean it up.
   gpr_mu_lock(&fd_freelist_mu);
   gpr_mu_unlock(&fd_freelist_mu);
   while (fd_freelist != nullptr) {