K 10
svn:author
V 6
rstone
K 8
svn:date
V 27
2010-11-20T04:54:58.706308Z
K 7
svn:log
V 647
MFC r212281 and 212282

In munmap() downgrade the vm_map_lock to a read lock before taking a read
lock on the pmc-sx lock.  This prevents a deadlock with
pmc_log_process_mappings, which has an exclusive lock on pmc-sx and tries
to get a read lock on a vm_map.  Downgrading the vm_map_lock in munmap
allows pmc_log_process_mappings to continue, preventing the deadlock.

Without this change I could cause a deadlock on a multicore 8.1-RELEASE
system by having one thread constantly mmap'ing and then munmap'ing a
PROT_EXEC mapping in a loop while I repeatedly invoked and stopped pmcstat
in system-wide sampling mode.

Approved by:	emaste (mentor)

END