fix request hang when flush the writing file

[xuensheng]

case： fd = open(filename) pid = fork() if(pid == 0) { sleep for a while close(fd) ---- call flush() } else if (pid > 0) { loop write(fd) ---- call write() }

flush holds the entry lock write holds the global lock and waits for the entry lock other requests wait for global lock, and all requests hang

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[abo969]

+1

[baiyubin]

OK