Date: Thu, 01 Nov 2007 12:48:50 +0100 From: Henrik Gulbrandsen <henrik@gulbra.net> To: freebsd-usb <freebsd-usb@freebsd.org> Subject: Patches for usb/46176 (problems with umass removal before unmount) Message-ID: <1193917730.1166.186.camel@Particle>
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--=-WJfY929iJC6/zoWlYhEg Content-Type: text/plain Content-Transfer-Encoding: 7bit This is not a complete fix, but this bug has been, eh... bugging me ever since I first crashed my system back in 2004. I had a set of patches for 5.3-BETA3, but it was too late to have it included on the 5.3 track, and since I was expecting some heavy redesign in the involved kernel parts, I dropped the patch delivery. Unfortunately, not that much has changed in this area in the past three years, and the current behavior is still far from the ideal I'd prefer: when you plug in a USB memory stick, a new directory is automatically mounted, and when it is unplugged, the directory silently disappears. The attached umass.tar.gz file contains patches and scripts that will give you this functionality in about 99% of the cases. The exact point of failure had moved a little since 5.3-BETA3, but the panic is still not difficult to avoid. However, the code is still highly experimental. In about 1% of USB detaches, the kernel will freeze. More specifically, in my four test runs, it happened after 63, 107, 6, and 321 cycles of attach followed by detach. Your mileage will most likely vary. I understand that the disk and file systems were not really designed for disappearing devices, so it's not that surprising if some lock is left locked under some circumstances. Or something completely different :-) Normally, I would continue looking for it, but I have other plans for the next month or two, so I don't know if I'll get the time. Perhaps someone else is interested enough to give it a shot... Anyway, here is a quick overview of the contents of umass.tar.gz: etc/usbd.conf.patch: an added entry in the usbd.conf file, to handle the umass devices. Yeah, I know... this should probably have used devd.conf instead, but this part is three years old and I didn't want to rerun the 321 attach/detach tests to verify that there is no important difference. etc/usbd/umass: the script that handles mounting and unmounting of the device directory under /media. I tried to make it work as expected, but I only have a single environment and a single USB memory stick to test with, so there may be some issues left. You'll get the gist, though... home/.umass: the main umass script will mount a directory if there is a single X11 user, or if there is no X11 user but a single user logged in at a local text terminal. The new directory will be owned by that user. If a .umass file exists in the user's home directory, it will be run to do more advanced things with the data. The included .umass is an example script that opens the GNOME file manager when the directory is mounted. src/msdosfs_vfsops.c.patch: the msdosfs will normally try to write the "clean" bit back to disk at unmount. When the drive is unplugged without warning, this would not only leave the file system marked as dirty, but will also cause a write failure. This patch modifies the implementation to keep the bit clean all the time if the file system is mounted with the "-o sync" option. This should be the case for USB memory sticks. src/usb_subr.c.patch: the usbd fails to match a usbd.conf entry during detach, since the detach event is filled in after the device is gone. This is easily fixed by moving a line to fill in the information before we clear all the relevant data. src/vfs_bio.c.patch: when the device is gone, remaining write attempts will correctly fail with ENXIO, but the failed buffer will get stuck in the sync queue. In 5.3-BETA3, this wasn't obvious until it blocked the unmounting of file systems at shutdown. Nowadays, it's detected earlier but this patch still seems to help. src/vfs_mount.c.patch: the only null-pointer problem left in the way of detaching is when the unmounting code discovers that it couldn't access the device and attempts to backtrack the correct handling already done. This patch makes it ignore the ENXIO error and finish the unmounting. That's it. I would hate to see another major release of FreeBSD where a system must be rebooted just because you forgot to do the umount before unplugging a umass device. It may again be too late to fix the remaining problems in time for the release, but I am at least hoping to see things work in 8-CURRENT Real Soon Now... /Henrik --=-WJfY929iJC6/zoWlYhEg--
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