mmap(2) MAP_FIXED isn't thread-safe (+testcase)

Hi all,

While investigating ports/115092 and other reports of seemingly random
page faults when running Wine, I think I've found the cause to be mmap
not being thread-safe when MAP_FIXED is used. It causes mmap(MAP_FIXED)
to return -1(ENOMEM) sometimes when it shouldn't, but also to return an
address with wrong protections, hence the protection faults occuring.

Attached is a test program that shows this. It runs two threads. The
first mmap()'s a region, starts a second thread and then goes in a loop
calling mmap(PROT_WRITE,MAP_FIXED) on that region, essentially
replacing that mapping. This is basically what rtld does to map an ELF
object for instance when dlopen(3) is called. The second thread tries
to steal the mapping from the first by calling mmap(PROT_NONE) in a
loop. After a while the program segfaults when the first thread tries
to write to the mapped region.

Some lines are commented out. If you remove the commenting, I hit on
the case where mmap(MAP_FIXED) returns -1.

The problem is in sys/vm/vm_mmap.c:vm_mmap(). In case of MAP_FIXED
first vm_map_remove() is called and then later vm_map_find(). This
would need some locking, but I don't know which lock or how to approach
this, so can somebody have a look at this?