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There is an order between covered vnode lock and allproc_lock, which is established by calling mountcheckdirs() while owning the covered vnode lock. mountcheckdirs() iterates over the processes, protected by allproc_lock. This order is needed and seems to be not avoidable. On the other hand, various VM daemons also need to iterate over all processes, and they lock and unlock user maps. Since unlock of the user map may trigger processing of the deferred map entries, it causes vnode locking to occur. Or, when vmspace is freed, dropping references on the vnode-backed object also lock vnodes. We get reverted order comparing with the mount/unmount order. For VM daemons, there is no need to own allproc_lock while we operate on vmspaces. If the process is held, it serves as the marker for allproc list, which allows to continue the iteration. Add _PHOLD_LITE() macro, similar to _PHOLD(), but not causing swap-in of the kernel stacks. It is used instead of _PHOLD() in vm code, since e.g. calling faultin() in OOM conditions only exaggerates the problem. Modernize comment describing PHOLD. Reported by: lists@yamagi.org Tested by: pho (previous version) Reviewed by: jhb Sponsored by: The FreeBSD Foundation MFC after: 3 week Approved by: re (gjb) Differential revision: https://reviews.freebsd.org/D6679
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