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 <merritt.a...@gmail.com> 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 <dil...@backplane.com> > 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 <merritt.a...@gmail.com> >> 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 >>> >> >> >
