Kernel global lock in sr.c slows down parallel operations to multiple drive

[Marc SCHAEFER <schaefer@alphanet.ch>]

Hello,

In jessie, the kernel had a very annoying bug: if you did I/O on
multiple sr devices, the global lock in sr.c would slow down
to a crawl.

E.g. 5 DVD-R written to in parallel gave the same performance as
one writing; and ejecting the DVD in parallel was completely
sequential.

Based on this:
   https://unix.stackexchange.com/questions/411735/why-does-linux-kernel-driver-sr-c-sr-block-ioctl-do-mutex-lock
and this:
   https://www.spinics.net/lists/linux-scsi/msg63706.html

I patched my kernel, and the performance augmented drastically,
you can watch the video here:

   https://www.youtube.com/watch?v=siDMzkRCTpQ&feature=youtu.be

or here:
   https://peertube.gaialabs.ch/videos/watch/7ad190bd-43d9-42dc-9336-984822db7cc3

I even think I reported that bug, but can't find it anymore.

However, it seems the kernel used by buster has the same issue. I am now
writing with BR drives (two of them) and I get abyssal performance.

Anyone can confirm ?

I will try to apply a similar patch and possibly report another bug.