Reminder: NET_NEEDS_GIANT, debug.mpsafenet going away in 7.0

Dear all:

This is a reminder e-mail that, in the very near future, Giant compatibility 
shims for network protocols will be removed.  These shimmed allowed Giant to 
be re-enabeld over the network stack as a result of linking in a service that 
required Giant (now all removed), or by setting the debug.mpsafenet variable 
to 1.  This means that the following will no longer be present:

   debug.mpsafenet sysctl
   debug_mpsafenet global variable
   NET_NEEDS_GIANT()
   NET_LOCK_GIANT(), NET_UNLOCK_GIANT(), NET_ASSERT_GIANT()
   NET_CALLOUT_MPSAFE

All instances of NET_{LOCK,UNLOCK,ASSERT}_GIANT() will be removed as they will 
no be no-ops.

The (unused) definition of NET_NEEDS_GIANT() will be removed.  The 
debug.mpsafenet sysctl and debug_mpsafenet global variable will be removed.

All instances of NET_CALLOUT_MPSAFE will be converted mechanically to 
CALLOUT_MPSAFE.

The *only* remaining case I am aware of where removing debug.mpsafenet 
presents an issue is credential-related firewall rules (uid, gid, jail).  I'm 
am currently in an active e-mail discussion with the various firewall 
maintainers about how to address this issue; as the implementations of these 
rules violate the global lock order, deadlocks occur if debug.mpsafenet isn't 
set to 1, which causes Giant to act as a guard lock preventing parallel lock 
acquisition in the firewall.  Hopefully we will have this resolved, in some 
form, soon.

I will remove the above in a series of commits; the only complicated bits are 
removing the NET_*_GIANT() calls from the socket system call code, where they 
made things quite a bit more complex than desirable due to additional error 
handling and unwinding, and in the Cronyx drivers, which interact with 
debug_mpsafenet in unusual ways (disabling their own locking when Giant is 
present).  Otherwise, this is a fairly low-risk change in practice, since 99% 
of FreeBSD users have been running without Giant over the network stack since 
5.4 (or was it 5.3?).

Robert N M Watson
Computer Laboratory
University of Cambridge