[haiku-bugs] Re: [Haiku] #10977: kdl in pthread_cond_wait on netsurf buildslave
- From: "bonefish" <trac@xxxxxxxxxxxx>
- Date: Tue, 24 Jun 2014 18:43:08 -0000
#10977: kdl in pthread_cond_wait on netsurf buildslave
-----------------------------+----------------------------
Reporter: pulkomandy | Owner: bonefish
Type: bug | Status: new
Priority: normal | Milestone: R1
Component: System/Kernel | Version: R1/Development
Resolution: | Keywords:
Blocked By: | Blocking:
Has a Patch: 0 | Platform: All
-----------------------------+----------------------------
Comment (by bonefish):
The immediate cause of the panic is just a missing
{{{
DEBUG_PAGE_ACCESS_END(context.page);
}}}
in `vm_soft_fault()` before [http://cgit.haiku-
os.org/haiku/tree/src/system/kernel/vm/vm.cpp?id=f7566626e5b389dbc90bc246660aeea8bad4f861#n4684
unlocking everything] when having to wait for a to-be-unmapped page to
become unwired.
Unfortunately that will leave another issue to be resolved: The page we
want to unmap isn't actually wired in this case. We have two threads that
want to wire the same virtual page for writing. The way `wire_page()`
works, they both first mark the respective address range wired before
calling `vm_soft_fault()` to map a writable page (there's only a readable
one from a lower cache). Either thread ignores its own pre-wired range,
but not that of the other thread. Hence the read-only page looks wired and
cannot be unmapped. Both threads would wait forever.
Not sure how involved a solution would be. It might be possible to mark
pre-wired ranges respectively and ignore them in `vm_soft_fault()`, but
this needs to be thought through thoroughly (particularly when to unmark
the ranges).
--
Ticket URL: <https://dev.haiku-os.org/ticket/10977#comment:1>
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