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SubjectRE: PROBLEM: I/O system call never returns if file desc is closed in the
Date

> 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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