Trying to grab locks during cngrab() when entering the debugger is
deadlock prone as all other CPUs are already halted (and thus unable
to release locks) when cngrab() is invoked. One could instead use
try-locks. However, the case that the try-lock fails still has to
be handled. In addition, if the try-lock works it doesn't provide
any greater ordering guarantees than is already provided by entering
and exiting DDB. It is simpler to define a simpler path for the
case that the try-lock would fail and always use that when entering
DDB. Messing with timers, etc. when entering DDB is dubious even if
the try-lock succeeds.
This patch attempts to use the smallest possible set of operations to
grab the vt(4) console when entering DDB without using any locks.
Reviewed by: emaste
Tested by: Matthew Macy
MFC after: 1 week
642ffab5fc