Removing Giant asserts from geom

[Alfred Perlstein]

On 5/19/16 2:31 PM, Konstantin Belousov wrote:
> On Thu, May 19, 2016 at 01:57:53PM -0700, Alfred Perlstein wrote:
>> OK, and why is thread0 needing Giant for so long?
> [Below is my opinion]
>
> It does not need Giant per se, it needs a work done to audit and turn it
> off. Probably most high profile subsystem in the kernel still relying on
> Giant is the newbus. Easy to see consequence is that handling thread0,
> that spends most of the time in discovering and configuring devices,
> actually needs to provide proper locking for the newbus.
>
> At least one attempt to do the later failed.  Troubles are both in the
> strange recursiveness of newbus, where device method could call into
> subr_bus.c, and in potential offloading of some work to other thread,
> which means that naive surround of the subr_bus.c methods with some
> global sleepable (and even recursive) lock would not work.
>
> Since newbus activity and early startup are rare events, fine-grained
> locking for them would result in, well, fine grained locking. There
> would be no break-through performance improvements after really hard
> work, including handling user reports for long time.
>
> So, it is not a priority for most people.
>
Thank you Konstantin, makes sense.

-Alfred