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Remove the volatile qualifier from busy_lock.

Authored by markj on Jul 28 2020, 3:08 PM.



Use atomic(9) to load the lock state. Some places were doing this
already, so it was inconsistent. In initialization code, the lock state
is still initialized with plain stores.

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markj requested review of this revision.Jul 28 2020, 3:08 PM
markj created this revision.
This revision is now accepted and ready to land.Jul 28 2020, 3:45 PM

(My questions are all unrelated to the direct purpose of the change.)

941 ↗(On Diff #75071)

I'm surprised to see an acquire here. The acquire performed by the sbusy should suffice.

986 ↗(On Diff #75071)

Why is a release used here? Note that the other case, where sharers > 1, does not use a release.

776–784 ↗(On Diff #75071)

Does anything that follows a "claim" call examine fields that are synchronized by the busy lock? I'm wondering about possible memory ordering issues.

246 ↗(On Diff #75071)

Documentation that all accesses should use atomic_*() would be good.

markj added inline comments.
986 ↗(On Diff #75071)

I would expect both cases to use a release store. Threads holding an sbusy lock may still modify page state (set valid or dirty bits, but not clear them).

776–784 ↗(On Diff #75071)

I don't think so. In all cases it's used immediately before freeing a page (or at least removing it from its object). Are you worried about issues like:

thread A, CPU 1:

<update valid or dirty state>
<hand off m to thread B>

thread B, CPU 2:

<observes old valid or dirty state>


Annotate the busy_lock field.

This revision now requires review to proceed.Jul 28 2020, 7:46 PM
246 ↗(On Diff #75071)

I simply added a locking annotation, but re-reading your comment makes me wonder if you were looking for something more descriptive.

246 ↗(On Diff #75071)

I think that the annotation suffices, but I would revisit the description/definition of "(A)". Specifically, the phrase "the field is atomic" is not very clear.

776–784 ↗(On Diff #75071)

Yes, exactly. I would suggest adding a comment to say that this function cannot be used in that way.

markj marked 2 inline comments as done.

Address feedback.

776–784 ↗(On Diff #75071)

I suppose we could introduce a vm_page_xbusy_disown(), akin to lockmgr_disown(), to provide any required synchronization if the need arises.

This revision is now accepted and ready to land.Jul 29 2020, 2:54 PM
This revision was automatically updated to reflect the committed changes.