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

[Daniel Eischen]

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.

-- 
DE