cvs commit: src/sys/nfsclient nfs_bio.c nfs_vfsops.c nfsargs.h
nfsmount.h src/sys/sys buf.h bufobj.h src/sys/kern vfs_bio.c
Brian Fundakowski Feldman
green at freebsd.org
Sun Jun 12 12:25:17 GMT 2005
On Sun, Jun 12, 2005 at 06:30:49AM -0400, Daniel Eischen wrote:
> On Sun, 12 Jun 2005, Alfred Perlstein wrote:
> > * Brian Fundakowski Feldman <green at freebsd.org> [050612 01:26] wrote:
> > > On Sun, Jun 12, 2005 at 01:08:33AM -0700, Alfred Perlstein wrote:
> > > >
> > > > Seriously, have you tested what happens to a libc_r app that
> > > > opens an nfs file F_SYNC? My guess is that it's not pretty.
> > >
> > > This code path is related to O_NONBLOCK, not O_FSYNC. O_FSYNC is
> > > synonymous with the slow fallback path that large transactional block
> > > now takes, rather than deadlocking. O_NONBLOCK really means that
> > > whatever they do, they are required to check for EAGAIN.
> > To make it perfectly clear.
> > If an application linked against libc_r opens a file with O_FSYNC.
> > Libc_r will set O_NONBLOCK (it does so for each open(2))
> > A write on that descriptor will return EAGAIN (to libc_r)
> > Libc_r will then attempt to select(2) on this decriptor, which
> > will return "ready" (as do all select(2)'s on disk files)
> > The question is:
> > Will Libc_r then busy spin?
> Yes, for the most part.
> > If so, how many other apps might get screwed just sometimes (over
> > nfs) because only _half_ of this "solution" is implemented?
> > Or is my thinking on this wrong?
> I think I agree with Alfred.
> If select() returns ready, then you should be able to write
> some part of your buffer. Also, anyone using a file descriptor
> in non-blocking mode should expect short writes and loop until
> the entire buffer has been written.
> >From my understanding, disk I/O has always returned ready, then
> blocked in the kernel if necessary. I think that if we are going
> to start honoring non-blocking mode for disk I/O (or NFS, whatever),
> it should be done fully. That means you allow short writes when
> select() returns ready, and select() doesn't return ready if no
> data can be written.
That's a good point. I wasn't more than vaguely aware of libc_r's
continued existance and usage of such things. Fixing select(3) to
match up would be easiest, right? Short writes are definitely not
allowed for a non-socket, though.
Brian Fundakowski Feldman \'[ FreeBSD ]''''''''''\
<> green at FreeBSD.org \ The Power to Serve! \
Opinions expressed are my own. \,,,,,,,,,,,,,,,,,,,,,,\
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