and dont_sleep_in_callout mutexes.  Also, disallow sleeping in hardware
interrupt handlers.

Approved by:	re (kensmith)

  Make callout_reset() return a non-zero value if a pending callout
  was rescheduled. If there was no pending callout, then return 0.

  Reviewed by:    iedowse, cperciva

Approved by:	re (kensmith)

list, set `curr_callout' to NULL. This ensures that we won't attempt
to cancel the current callout if the original callout structure
gets recycled while we wait to acquire Giant.

This is reported to fix an intermittent syscons problem that was
introduced by revision 1.96.

callout is first initialised, using a new function callout_init_mtx().
The callout system will acquire this mutex before calling the callout
function and release it on return.

In addition, the callout system uses the mutex to avoid most of the
complications and race conditions inherent in asynchronous timer
facilities, so mutex-protected callouts have much simpler semantics.
As long as the mutex is held when invoking callout_stop() or
callout_reset(), then these functions will guarantee that the callout
will be stopped, even if softclock() had already begun to process
the callout.

Existing Giant-locked callouts will automatically pick up the new
race-free semantics. This should close a number of race conditions
in the USB code and probably other areas of the kernel too.

There should be no change in behaviour for "MP-safe" callouts; these
still need to use the techniques mentioned in timeout(9) to avoid
race conditions.

places where it occurs, not just one. :-)

Pointed out by:	glebius
Pointy had to:	cperciva

i.e., only clear c->c_func if the callout c is being used via the old
timeout(9) interface.

Requested by:	glebius

callout_stop, callout_drain, and callout_deactivate, but is not
automatically cleared when a callout returns.

the first column is performed using /**, not /*-.

with Giant or not, and include the function point so it can be looked
up against the kernel symbol table during trace analysis.

callout_reset rather than calling callout_stop.  This results in a few
lines of code duplication, but it provides a significant performance
improvement because it avoids recursing on callout_lock.

Requested by:	rwatson