On Mon, Sep 22, 2014 at 10:12 AM, Jonathan Corbet <corbet@xxxxxxx> wrote: > On Fri, 19 Sep 2014 13:33:14 -0400 > Milosz Tanski <milosz@xxxxxxxxx> wrote: > >> > - Non-blocking I/O has long been supported with a well-understood set >> > of operations - O_NONBLOCK and fcntl(). Why do we need a different >> > mechanism here - one that's only understood in the context of >> > buffered file I/O? I assume you didn't want to implement support >> > for poll() and all that, but is that a good enough reason to add a >> > new Linux-specific non-blocking I/O technique? >> >> I realized that I didn't answer this question well in my other long >> email. O_NONBLOCK doesn't work on files under any commonly used OS, >> and people have gotten use to this behavior so I doubt we could change >> that without breaking a lot of folks applications. > > So I'm not contesting this, but I am genuinely curious: do you think > there are applications out there requesting non-blocking behavior on > regular files that will then break if they actually get non-blocking > behavior? I don't suppose you have an example? > > Thanks, > > jon Earlier in this thread Jeff pointed ( https://lkml.org/lkml/2014/9/15/942 ) to a bug in RH bugzilla ( https://bugzilla.redhat.com/show_bug.cgi?id=136057 ) when an application (squid) reads regular disk files started returning EAGAIN when read from (provided that they were open with O_NONBLOCK) and since that doesn't cause readhead it spins on it forever. As far as I know O_NONBLOCK for regular files in Linux is undefined behavior as non of the man pages I looked at (esp. fnctl, 2 open, 3 open) specify what happens in the case of non-network, non-fifo case (some of them refer to file descriptors that support non-blocking operation, which is pretty vague). So even if squid is wrong in it's behavior (since it's undefined), a quick google search reveals lots of mailing lists / forum posts of people essentially describing the behavior to date. Eg. O_NONBLOCK on regular files blocks, with select/poll/epoll always returning a ready behavior. Based on that anecdotical evidence, I assume a decent chunk of user apps would beak. - Milosz -- Milosz Tanski CTO 16 East 34th Street, 15th floor New York, NY 10016 p: 646-253-9055 e: milosz@xxxxxxxxx -- 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
