2.4.1-ac5 - The loopback hang saga continues

From: Byron Stanoszek (gandalf@winds.org)
Date: Wed Feb 07 2001 - 17:45:13 EST

It appears that the loopback-hang parasite is alive and well in 2.4.1-ac5.
I've done several tests and I thus provide the following information:

The bug is independent of UP or SMP configured.. it hung both ways, but the
box itself is UP.

It appears to hang when internal buffers get filled. The way I see it, copying
files from disk to the loopback device (which is a file on the same disk)
begins to read from the disk. When the internal read buffer is full, the
kernel's queued writes start activating and the data gets copied to the
loopback file. This process repeats over and over, as it should normally.

Sometimes however, during a read from the disk, it fills up its buffers and
then never makes the accompanying write. In fact, the entire device freezes on
the read.

I was able to lessen the frequency of hanging by using the -v flag and tapping
^S and ^Q to temporarily 'pause' copying. This ensures that the read buffer
will never become full to the point where it could cause the hang, and appears
to work -- until it came across the libc.a file. There was no way to pause it
here because nothing is being outputted to the screen while it's copying
libc.a. Unfortunately, it fills the buffer too quick and hangs 100% every time.
The disk is totally nonresponsive at this point, and a hard reset is necessary.

I hope this helps anyone who is still tracking down the loopback problem.


Byron Stanoszek                         Ph: (330) 644-3059
Systems Programmer                      Fax: (330) 644-8110
Commercial Timesharing Inc.             Email: byron@comtime.com

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