shared lwkt locks

Alex Merritt merritt.alex at
Thu Jul 16 18:25:48 PDT 2015


Thanks. I am working with the locks in the order as you described. I am
specifically trying to understand if the order also applies to shared lock
acquisitions, as I am experiencing a panic when I drop a lock the second
time, after having performed two shared acquisitions:

vm_object_drop(A) // <-- panic

The panic is here:

Is this panic expected behavior (i.e. does a release relinquish any number
of prior shared lock acquires to a token)?

The location of the warning in comments I am referring to is here:


On Thu, Jul 16, 2015 at 5:54 PM, Matthew Dillon <dillon at>

> 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() ?
> -Matt
> On Thu, Jul 16, 2015 at 5:31 PM, Alex Merritt <merritt.alex at>
> wrote:
>> 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the Kernel mailing list