shared lwkt locks
merritt.alex at gmail.com
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: http://bxr.su/DragonFly/sys/vm/vm_object.c#332
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 backplane.com>
> 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 gmail.com>
>> 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.
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