Date: Tue, 9 Sep 2003 19:46:12 -0400 (EDT) From: Daniel Eischen <eischen@vigrid.com> To: Dan Langille <dan@langille.org> Cc: Nate Lawson <nate@root.org> Subject: Re: comments on proposed uthread_write.c changes Message-ID: <Pine.GSO.4.10.10309091929500.13114-100000@pcnet5.pcnet.com> In-Reply-To: <3F5B89B3.11367.112C1E2D@localhost>
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Synopsis:
libc_r wraps write() and changes a blocking file descriptor
to non-blocking behind the scenes. When __sys_write()
returns less than the number of bytes requested, libc_r's
write() continues looping trying to finish the requested
write.
Problem:
A return of 0 for a write() to a tape device means end-of-file.
Libc_r's write treats a return of 0 from __sys_write() as a
partial write and continues looping. The end-of-file is
passed and a few more blocks may be written before the
physical end-of-tape is reached.
[ In this context, end-of-file means a the shiny end-of-tape
marker on the tape (at least that's what it looks like on
7 & 9 track tapes -- am I dating myself?). It doesn't mean
an EOF marker. ]
I've been reading the POSIX spec in regards to write():
http://www.opengroup.org/onlinepubs/007904975/toc.htm
particularly the Rationale at the end. I think it is OK
for libc_r to special case a return of 0 from __sys_write()
and pass it back to the caller.
Any comments?
On Sun, 7 Sep 2003, Dan Langille wrote:
> On 7 Sep 2003 at 12:32, Daniel Eischen wrote:
>
> > On Sun, 7 Sep 2003, Dan Langille wrote:
> >
> > > A problem with pthreads and EOT has been identified. See PR 56274. It
> > > was suggested the solution was probably just a matter of changing one of
> > > the >0 tests to >=0 in uthread_write.c
> > >
> > > Any comments on that?
> >
> > I don't know that a return of 0 isn't valid for other devices.
> > If this is the case, a return of 0 for blocking writes may break
> > other applications.
> >
> > The patch isn't quite correct (at least looking at -current srcs).
> > Lines 98-99 are:
> >
> > if (blocking && ((n < 0 && (errno == EWOULDBLOCK ||
> > errno == EAGAIN)) || (n >= 0 && num < nbytes))) {
> >
> > This will get entered first if n == 0, and I don't think your
> > proposed patch would have any effect. I think you would have
> > to change the "n >= 0" above to be "n > 0" in conjunction with
> > your patch.
>
> Ahh thank you. That explains why the test results with the original
> patch did not differ from -STABLE or 5.1-RELEASE. After adding your
> suggestions, we have had success.
>
> The points to note:
>
> 1. The status that stopped the writing was 0
> 2. It wrote 17,256 blocks, and read 17,256 blocks.
>
> Point 1 is key to determining EOT. Point 2 is what you always want
> to have...
>
> [dan@undef:~/tape-test] $ sudo ./tapetest /dev/nsa0
> *rewind
> Rewound /dev/nsa0
> *rawfill
> Begin writing blocks of 64512 bytes.
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> +++++
> weof_dev
> Wrote EOF to /dev/nsa0
> Write failed. Last block written=17256. stat=0 ERR=Unknown error: 0
> *rewind
> Rewound /dev/nsa0
> *scan
> Starting scan at file 0
> 17256 blocks of 64512 bytes in file 0
> End of File mark.
> End of File mark.
> End of tape
> Total files=1, blocks=17256, bytes = 1113219072
> *
>
> > This could be solved at the application level by selecting on
> > the tape device and performing non-blocking writes to it. Since
> > the application knows that a return of 0 is end-of-tape, it
> > must also know the difference between talking to a tape device
> > and talking to a regular file, socket, etc.
>
> True. But *if* the code is wrong, it should be fixed.
>
> FWIW, the patch follows. As always, opinions and suggestions are
> welcome.
>
> --- uthread_write.c.org Sun Sep 7 10:58:31 2003
> +++ uthread_write.c Sun Sep 7 15:41:34 2003
> @@ -93,7 +93,7 @@
> * write:
> */
> if (blocking && ((n < 0 && (errno == EWOULDBLOCK ||
> - errno == EAGAIN)) || (n >= 0 && num < nbytes))) {
> + errno == EAGAIN)) || (n > 0 && num < nbytes))) {
> curthread->data.fd.fd = fd;
> _thread_kern_set_timeout(NULL);
>
> @@ -131,7 +131,7 @@
> * If there was an error, return partial success
> * (if any bytes were written) or else the error:
> */
> - } else if (n < 0) {
> + } else if (n <= 0) {
> if (num > 0)
> ret = num;
> else
>
> --
> Dan Langille : http://www.langille.org/
>
--
Dan Eischen
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