From: Al Viro
> Sent: 04 November 2015 16:28
> On Wed, Nov 04, 2015 at 03:54:09PM +0000, David Laight wrote:
> > > Sigh... The kernel has no idea when other threads are done with "all
> > > io activities using that fd" - it can wait for them to leave the
> > > kernel mode, but there's fuck-all it can do about e.g. a userland
> > > loop doing write() until there's more data to send. And no, you can't
> > > rely upon them catching EBADF on the next iteration - by the time they
> > > get there, close() might very well have returned and open() from yet
> > > another thread might've grabbed the same descriptor. Welcome to your
> > > data being written to hell knows what...
> >
> > That just means that the application must use dup2() rather than close().
> > It must do that anyway since the thread it is trying to stop might be
> > sleeping in the system call stub in libc at the time a close() and open()
> > happen.
>
> Oh, _lovely_. So instead of continuation of that write(2) going down
> the throat of something opened by unrelated thread, it (starting from a
> pretty arbitrary point) will go into the descriptor the closing thread
> passed to dup2(). Until it, in turn, gets closed, at which point we
> are back to square one. That, of course, makes it so much better -
> whatever had I been thinking about that made me miss that?
Do I detect a note of sarcasm.
You'd open "/dev/null" (lacking a "/dev/error") and use that as the source fd.
So any writes (etc) would be discarded in a safe manner, and nothing will block.
David
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