Re: shared lwkt locks

That warning is in comments, it's documenting the fact that
vm_object_drop() might be called reentrantly due to potentially being
locked shared.

-Matt

On Thu, Jul 16, 2015 at 9:40 PM, Matthew Dillon <[email protected]>
wrote:

> You are doing something more than what you coded above and it is causing
> the object's hold count to go negative on the second drop.  That particular
> panic is not related to the token itself.   It looks to me like the object
> must have been dropped more times than it was held.
>
> -Matt
>
> On Thu, Jul 16, 2015 at 6:25 PM, Alex Merritt <[email protected]>
> wrote:
>
>> Matt,
>>
>> 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_hold_shared(A)
>> vm_object_hold_shared(A)
>> vm_object_drop(A)
>> 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:
>>
>> http://bxr.su/DragonFly/sys/vm/vm_object.c#320
>>
>> -Alex
>>
>>
>> On Thu, Jul 16, 2015 at 5:54 PM, Matthew Dillon <[email protected]>
>> wrote:
>>
>>> 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 <[email protected]>
>>> 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
>>>>
>>>
>>>
>>
>