destroy_dev_sched*: Don't hold Giant for all deferred destroy_dev.

[jhb]

Summary

Rather than using taskqueue_swi_giant which holds Giant for all
deferred destroy_dev calls, create a separate queue for destroyed
devices with D_NEEDGIANT set in the corresponding cdevsw. The task
for this queue holds Giant whild destroying deferred devices while the
task for the default queue does not hold Giant.

In addition, switch to taskqueue_thread for destroy_dev_sched.
Deferred destroy_dev requests don't need to run at an SWI priority.

[markj]

sys/kern/kern_conf.c

1454

Does Giant need to be held across destroy_devl() too? That function invokes d_purge (seems to be mostly unused) and cdevpriv destructors (not sure if D_NEEDGIANT implies that they're synchronized by Giant too).

[jhb]

sys/kern/kern_conf.c

1454

Oh, hmm. Looks like d_purge isn't handled by the gianttrick hack, and yeah, cdevpriv probably needs Giant as well.

[jhb]

Hold Giant over destroy_devl as well.

[jhb]

sys/kern/kern_conf.c

1452

Sigh, so this is a LOR with devmtx in dev_lock I think.

I would need to push Giant down into destroy_devl() instead along with the previous version of this patch I think.

[jhb]

sys/kern/kern_conf.c

1452

That doesn't work out so well because dev_lock is still held when d_purge is invoked.

I guess fixing this for real would require separate global lists and tasks.

[jhb]

• Switch to a separate queue + task.