[haiku-bugs] Re: [Haiku] #3685: PANIC: vnode already exists
- From: "bonefish" <trac@xxxxxxxxxxxx>
- Date: Wed, 26 Jun 2013 14:21:16 -0000
#3685: PANIC: vnode already exists
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Reporter: rossi | Owner: axeld
Type: bug | Status: new
Priority: normal | Milestone: R1
Component: System/Kernel | Version: R1/Development
Resolution: | Keywords:
Blocked By: | Blocking: 5260
Has a Patch: 0 | Platform: All
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Comment (by bonefish):
I just ran into a similar `panic()` that I was able to reproduce and
analyze. The cause in my case was a race condition with the VFS's
`get_vnode()` in response to `_user_open_dir_entry_ref()`. Since
`get_vnode()` first calls `create_new_vnode_and_lock()` and afterwards the
FS's `get_vnode()` hook, there's a time window where the vnode exists, but
the respective node may not exist in the FS. If the node doesn't actually
exist and concurrently another thread creates a new file or directory with
that node ID, the `new_vnode()` or `publish_vnode()` it calls will
encounter the existing vnode and fail (the former will `panic()`).
Not sure what a good solution would be. `new_vnode()`/`publish_vnode()`
could wait until the vnode becomes unbusy or disappears (a new marker flag
might be needed to distinguish the case from others where the node is
busy). That might be problematic, however, since the caller may hold a FS
specific lock, so that we'd risk a deadlock with the concurrent
`get_vnode()` hook which may also want to acquire that lock.
An IMO cleaner alternative would be to change the interface and semantics
of the `get_vnode()` hook. It would be required to create the vnode (via
`publish_vnode()` or a new interface). We would have to push the burden of
dealing with concurrent invocations to the hook, though.
Note that this ticket may actually point to a different problem than the
one I've analyzed. In my case `vnode->node` was expectedly NULL. Here it
isn't, though.
--
Ticket URL: <http://dev.haiku-os.org/ticket/3685#comment:3>
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