scsi/sr - Impossible to write CDs/DVDs simultaneously (since BKL removal ?)

When burning multiple CDs/DVDs in parallel with multiple SATA drives, you can notice that :
- the process is abnormaly slow using cdrskin or wodim (top shows lots of I/O wait),
- the discs are burnt one after each other using cdrecord.

A simpler way to touch the problem is running :

$ eject /dev/sr0 & eject /dev/sr1
drives ejects sequentialy, rather than simultaneously as we were used to.

I think this problem arose after the Big Kernel Lock removal.
As far as I can understand the new "sr_mutex" private mutex in drivers/scsi/sr.c generates a per-driver locking, where a per-device locking would have been far better. I also think other drivers like sg are impacted by the same problem.

I am absolutely unable to arrange this, and would be pleased if someone competent could work on this.

Please CC me when answering as I'm not subscribing to the list.

Yours,
Étienne DADA