Date: Sat, 29 May 1999 01:46:44 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: hackers@FreeBSD.ORG Subject: Possible race in pipe device driver, esp on multi-cpu machines. Message-ID: <199905290846.BAA29926@apollo.backplane.com>
next in thread | raw e-mail | index | archive | help
A friend of mine upgraded one of his machines to a duel-cpu
box and upgraded the OS to -STABLE, and he noticed that his
backups were being corrupted. The corruption appears to occur when
he transfers huge gzip'd tar files over a 100BaseTX network:
rsh remote -n "cat remotefile" > localfile
ssh remote -n "cat remotefile" > localfile
rcp remote:remotefile localfile
scp remote:remotefile localfile
The remotefile in this case is a huge 192MB gzip'd tar file. Portions
of the file get corrupted - generally the corruption consists of a small
sequence of 8 or so bytes at a random offset in the file being repeated
twice, destroying data that should have been sent instead. The corrupted
file winds up being the same size as the original file, but with
occassional repeating patterns.
Through experimentation we determined that it was NOT the TCP connection,
programs run, or filesystem that are introducing the corrupted data.
I believe that the problem is situated in the pipe device driver. It
should also be noted that one of my friend's machines is a duel-cpu
machine. Both are running FreeBSD-STABLE. If I replace the "cat" in
the rsh/ssh with a "dd bs=4k", the corruption goes away. I think the
problem is occuring on the duel-cpu machine by creating a window of
opportunity due to the way processes are scheduled that is not being
protected by the pipe device.
I seem to recall that the pipe device tries to optimize certain situations
when a reader is blocked waiting for input. This code is my primary
suspect at the moment.
We are attempting to reproduce the problem with a smaller dataset, but
if anyone is hot on the pipe code in the kernel and can give it a once-over
we may be able to find the bug more quickly.
-Matt
Matthew Dillon
<dillon@backplane.com>
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199905290846.BAA29926>