callout(9) and Giant lock

[John Baldwin]

On Sunday 28 June 2009 11:57:05 am Robert Watson wrote:
> 
> On Sun, 28 Jun 2009, Sebastian Huber wrote:
> 
> > suppose that a certain time event triggered several callout functions. What 
> > happens if the first of these callout functions blocks on the Giant lock? 
> > Does this delay all further callout functions until the Giant lock is 
> > available for the first callout function? What happens if one of the callout 
> > function blocks forever? Does this deadlock the system?
> 
> Callouts are marked as MPSAFE or non-MPSAFE when registered.  If non-MPSAFE, 
> we will acquire Giant automatically for the callout, but I believe we'll also 
> try and sort non-MPSAFE callouts behind MPSAFE ones in execution order to 
> minimize latency for MPSAFE callouts.  Most callouts acquire locks of some 
> sort, and stalling any callout indefinitely will stall the entire callout 
> thread indefinitely, which in turn could lead to a variety of odd behaviors 
> and potentially (although not necessarily) deadlock.

FWIW, we do not actually sort the callouts in this manner, so all callouts
will be blocked until Giant is acquired.

-- 
John Baldwin