Messages in this thread |  | | | From | "David Schwartz" <> | | Subject | RE: PROBLEM: I/O system call never returns if file desc is closed in the | | Date | Thu, 7 Jun 2001 15:14:04 -0700 |
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> At 22:35 +0100 2001-06-06, Alan Cox wrote:
> > > This report describes a problem in the usage of file > > > descriptors across
> >> multiple threads. When one thread closes a file descriptor, another > >> thread which waits for an I/O on that file descriptor is not notified > >> and blocks forever.
> >THe I/O does not block forever, it blocks until completed.
> That's still "forever" if you don't specify a timeout in the select.
If you don't want to block until an operation completes, then don't ask to!
> >The actual final > >closure of the object occurs when the last operation on it exits
> Select is defined as to return, with the appropriate bit set, if/when > a nonblocking read/write on the file descriptor won't block. You'd > get EBADF in this case, therefore causing the select to return would > be a Good Thing.
That is not quite correct. That is a good approximate definition of 'select's behavior, but it is not exact. As for your assertion that a noblocking read/write wouldn't block, that's not necessarily true. Remember, the 'close' may not have taken affect yet, since the descriptor is still in use by virtue of being selected on.
> A related problem is that the second thread my be inside a blocking > read() instead of a select() call. It'd never continue. :-(
Perfect. Doing this is absolutely, positively wrong and the more you are punished for it, the better. It's as wrong as calling 'free' on a chunk of memory when another thread may be usign it. It is impossible to make this work safely, as another thread could open a socket or file and get the same descriptor write before the call to 'read' is entered. There's no possible way to do this because there is no 'unlock a mutex and read' operation.
> HOWEVER: IMHO it's bad design to distribute the responsibility for > file descriptors between threads.
Why? That's a great design and it's absolutely essential in many cases. Suppose, for example, I have two descriptors I want to write to. If I assign one thread to each socket permanently, then I'm 100% guaranteed a context switch every time I change which socket I'm writing to, so if there's lots of small bits of data going out both of them, my performance will suck. But if I assign one thread to both socket descriptors, I'm guaranteed that one connection will stall if the the application-level send queue for the other has been swapped out to disk. Not distributing network I/O across threads dynamically is a recipe for either low performance or bursty performance.
DS
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