Add function to OSD to get values without taking the lock.

[stevek]

Summary

There are some cases of OSD use where the value is only initialized once
at a point where successive access of the value can be done so safely
without the need to take the lock.

Obtained from: Juniper Networks, Inc.

[markj]

Some uses of OSD may do so in a way where the value is only set once at a point where successive get of the value can be done so safely without the need to take the lock.

I think this sentence is not quite right. :)

sys/kern/kern_osd.c

281

osd_get_unlocked() would be more consistent with this kind of pattern elsewhere in the tree.

291

[stevek]

Changed from _nolock to _unlocked in naming and updated commit log message.

[markj]

sys/kern/kern_osd.c

291

Assuming you just didn't see it, I think this suggestion should still be applied. Or is there some reason not to?

[stevek]

sys/kern/kern_osd.c

291

I probably would not be necessary in the cases that I have seen where unlocked access makes sense. However, I suppose one could do this for some of the potential cases, but wouldn't atomic_store_ptr be necessary also in the osd_set_reserved() function if doing so?

[markj]

sys/kern/kern_osd.c

291

It's not strictly necessary. Adding the atomic qualifier here 1) helpers readers understand that this isn't a normal, synchronized memory access, 2) provides more information to concurrency sanitizers to help avoid reporting false positives.

I think it would make sense to modify osd_set_reserved() too.

[stevek]

Use atomic to read OSD slot data