USB dead device fix

I have a slug (NSLU2) at home, serving out a big USB hard disk. I like my slug. It's small, and it runs Debian, and so long as I remember that it's really small, it runs really well.

It looks like USB drives suffer from lots of issues with dodgey firmware and drivers. Certainly, I haven't escaped them. Every few months I lose the USB drive:
kern.log:

Apr 12 20:32:54 slug kernel: scsi 1:0:0:0: rejecting I/O to dead device

...

This can cause filesystem corruption as well, and is generally "A Bad Thing". Fixing this issue is (for me) a case of remounting the filesystem. I recall once I also had to cycle the power too. Fortunately, I found this page: http://www.mail-archive.com/linux-usb-users@lists.sourceforge.net/msg16510.html which has a very nifty technique for coaxing the USB stack back to life.

I've written a script to pull out the sysfs name of the USB device for a given mount point. At some point I'll add this in to my slug so that it "just works". That just leaves a log scraper to pick up each mount and get the sysfs name, wait for a failure, unmount the dead file system, rebind the USB device, check the file system for damage, and remount the filesystem. Easy!

#!/bin/bash
# get_usb_name.sh 
# e.g.: ./get_usb_name.sh /media/usb2

USB_DEV=$(mount | grep ${1} | cut -f 1 -d ' ' | sed -e 's/[0-9]$//' -e 's(^/dev/((')
USB_ADDR=$(ls -l /sys/block/${USB_DEV}/device | sed -e 's(/host.*$((' -e 's(^.*/((')
if [ -e /sys/bus/usb/drivers/usb-storage/${USB_ADDR} ]
then
echo $USB_ADDR
else
echo "could not verify usb address of ${USB_ADDR} for device ${USB_DEV}" >&2
fi

Perl threads :-(

I chucked in an entry for the Computer languages shootout game for the Perl implementation of a thread-ring. I've a few observations:

1. My application relies on semaphores that unblock themselves quickly. On average, there's two threads running (rather than blocking on a semaphore) at a time, but my dual core PC is only 60-80% utilised. Something in the cond_wait implementation is sleeping.
2. Try as I might, semaphores were the only IPC mechanism I could get to work in a way that appeared "correct" (rather than spurious errors about shared or locked, or whatever). This may be a reflection on the workman rather than his tools.
3. Getting any cross-platform, efficient, thread-safe, IPC across muliple versions of Perl, is like pulling teeth. I've got a blended semaphore-based implementation ready to go. By blended, I mean it uses (inefficient) semaphores for the tricky first lap around the ring (ensuring that all initialisation is done correctly), and then move over to "something" immediate for message passing. I need a working message passing implementation to slot in now.
4. As the shootout game demonstrates, Perl threads are not even close to universally available in any sense of the term.
   

My entry: http://shootout.alioth.debian.org/gp4/benchmark.php?test=threadring&lang=perl&id=0

I'm going to see if libevent holds any hope for me. If not, then maybe a mass of file handles, and select statements...