Running Slackware64-current on an IBM x3650 server, after upgrading system to kernel version 4.19.5, I am no longer able to write data to my LTO tape drives. Using a Dell PowerVault 132t connected to the server via an Adaptec ASC-29320ALP U320 card (aic79xx module). I can run the mtx command to load/unload and move tapes around fine, and the mt command also works fine with accessing tapes in either of the LTO2 drives. However, if I run the command "tar -cvf /dev/st0 /path/to/backup", the system acts like it is writing the files to tape, but nothing seems to get actually written except filemarks. Running a "tar -tvf /dev/st0" immediately exits. Reverting back to the Slackware64-current kernel-generic-4.19.4 package and everything works fine again. This same thing is happening on a second Slackware64-current IBM workstation, with an Adaptec AHA-2940U2/U2W card (aic7xxx module), accessing a second PowerVault 132t tape library. There are no logs generated in /var/log/syslog or /var/log/messages on either system. Latest kernel tested is 4.19.8. If I write data to tape with 4.19.4, I am still able to read it with the newer kernels, but data written with 4.19.5 or higher cannot be read.
Can you check whether reverting commit f3587d76da05f68098ddb1cb3c98cc6a9e8a402c helps, and if that doesn't help, run a bisect between kernel versions v4.19 and v4.20-rc5?
I can confirm that reverting commit f3587d76da05f68098ddb1cb3c98cc6a9e8a402c does indeed solve the problem.
A fix has been queued for the 4.19 stable series and should be included in a 4.19.x stable kernel soon. See also
Just installed 4.19.11, and the fix works. Many thanks to everyone who helped squash this bug. :-)