When and when not to use CTLFLAG_MPSAFE with the SYSCTL macros..?

[John Baldwin]

On Wednesday, August 05, 2015 08:38:18 AM Hans Petter Selasky wrote:
> On 08/05/15 00:47, Garrett Cooper wrote:
> > Hi,
> > 	I’ve been trying to figure out (because sysctl(9) is lacking) when to use CTLFLAG_MPSAFE. Is it strictly when dealing with SYSCTL_PROC handlers that do proper locking of shared resources, or are there other nuances that need to be handled?
> > 	I’m also asking because SYSCTL_UQUAD, for instance, explicitly uses CTLFLAG_MPSAFE in the handler, which is a bit confusing.
> > Thanks!
> > -NGie
> 
> Hi,
> 
> This flag decides if you have Giant automatically locked or not around 
> the sysctl proc. Your functions should have their own locks basically. I 
> believe it is a leftover from many years ago, when the FreeBSD kernel 
> was going multi threaded.

It's only about 2-3 years old actually.

To answer your question Garrett: yes it is really only for SYSCTL_PROC
handlers.  The existing "simple" handlers like sysctl_handle_int are
as atomic as they can be regardless of Giant, so they don't need Giant.

If you have a variable that you want to control access to via locking you
need to use a custom handler, even if it is a simple int.

-- 
John Baldwin