device drivers to declare that the ifp->if_start() method implemented
by the driver requires Giant in order to operate correctly.
Add a 'struct task' to 'struct ifnet' that can be used to execute a
deferred ifp->if_start() in the event that if_start needs to be called
in a Giant-free environment. To do this, introduce if_start(), a
wrapper function for ifp->if_start(). If the interface can run MPSAFE,
it directly dispatches into the interface start routine. If it can't
run MPSAFE, we're running with debug.mpsafenet != 0, and Giant isn't
currently held, the task is queued to execute in a swi holding Giant
via if_start_deferred().
Modify if_handoff() to use if_start() instead of direct dispatch.
Modify 802.11 to use if_start() instead of direct dispatch.
This is intended to provide increased compatibility for non-MPSAFE
network device drivers in the presence of Giant-free operation via
asynchronous dispatch. However, this commit does not mark any network
interfaces as IFF_NEEDSGIANT.
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