shared lwkt locks

Matthew Dillon dillon at
Thu Jul 16 17:54:37 PDT 2015

Lock releases must match lock acquisitions.  lwkt_tokens have an additional
requirement that lock releases must be performed in exactly the reverse
order as the acquires.  So if you acquire A, B, C, B, X, Y  you have to
release in reverse, Y, X , B, C, B, A.

Shared tokens are very dangerous and must be used carefully.  The vm_object
code is the most complex code in the kernel that uses shared tokens, and as
you can see by reading it, it can get nasty because a particular token
cannot mix shared and exclusive use in the same thread.

I'm not sure which warning you are talking about, where in vm_object_drop()


On Thu, Jul 16, 2015 at 5:31 PM, Alex Merritt <merritt.alex at>

> Hi,
> Does releasing a lwkt_token release all prior gettoken_shared invocations
> to that lock, or is one required to perform reltoken as many times as
> you've performed gettoken_shared? There is a warning in vm_object_drop that
> the lock may be shared -- I was wondering what exactly that warning is
> meant to warn against. Apologies if the answer is apparent somewhere.
> Thanks,
> Alex
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