On Fri, Jul 20, 2007 at 12:10:00AM -0400, Jan Harkes wrote: > I will try to find a clean way to block the close syscall until fput > drops the last reference. However I realize that such an implementation > would not be acceptable for other file systems, and there are some > interesting unresolved details such as 'which close is going to be the > last close'. Simply impossible. fd = open(...); fd2 = dup(fd); .... close(fd2); <-- deadlock close(fd); Or fd = open(...); p = mmap(..., fd, ...); close(fd); <-- deadlock ... munmap(p, ...); The problem is that you are mixing two different operations: * closing descriptor: descriptor doesn't refer to this opened file anymore. Can be done by close(), can be done by dup2(), can be done by fcntl() analog of dup2(), can be done by execve(), can be done by exit(). And can be done by death-by-signal. * closing opened file: no references (including descriptors) exist anymore, no IO of any kind will be done on that IO channel. Can happen when descriptor gets closed, can happen when mapping gets destroyed (munmap, etc.), can happen when SCM_RIGHTS datagram gets destroyed (including the garbage collection), can happen when any syscall on that file finishes after another thread has closed descriptor passed to that syscall, etc. You _can't_ expect the errors from the latter to be reliably passed to some process; for one thing, many files can get closed by single system call (e.g. closing an AF_UNIX socket with pending SCM_RIGHTS datagrams will flush those datagrams, dropping references to files we tried to pass via them). Why does CODA need special warranties in that area, anyway? Related question: does fsync() force the writeback? - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
