callout(9) and Giant lock

[John Baldwin]

On Monday 13 July 2009 2:49:24 pm Robert N. M. Watson wrote:
> 
> On 13 Jul 2009, at 19:17, John Baldwin wrote:
> 
> >> 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.
> 
> I must have been remembering a proposed change -- as you say, it's  
> certainly not in kern_timeout.c. However, I'd rather just eliminate  
> support for Giant in callouts in 9.x than try to further facilitate  
> them :-)

Agreed. :)

-- 
John Baldwin