On Mon, 26 May 2008, Robert Watson wrote:
In the continuing campaign to eliminate the Giant lock from the dregs of the
network stack, I thought I'd send out a list of non-MPSAFE netisr handlers:
Location Handler Removed with IFF_NEEDSGIANT
dev/usb/usb_ethersubr.c:120 usbintr Yes
net/if_ppp.c:277 pppintr Yes
netinet6/ip6_input.c ip6_input No
The plan for 8.0 is to remove the NETISR_MPSAFE flag -- all netisr handlers
will be executed without the Giant lock. This doesn't prohibit acquiring
Giant in the handler if required, although that's undesirable for the
obvious reasons (potentially stalling interrupt handling, etc). Obviously,
what would be most desirable is eliminating the remaining requirement for
Giant in the IPv6 input path, primarily consisting of mld6 and nd6.
With this in mind, my current plan is to remove the flag and add explicit
Giant acquisition for any remaining handlers in June when IFF_NEEDSGIANT
device drivers are disabled.
I've now removed the NETISR_MPSAFE flag -- all netisr handlers are now assumed
to DTRT with respect to locking. At least until usb and ppp are sorted out,
I've introduced NETISR_FORCEQUEUE as an interim measure, which allows
protocols to request that they always operate the deferred dispatch, meaning
they can acquire Giant if they need to, and modified those two to do so.
That should go away by 8.0 also.
Robert N M Watson
Computer Laboratory
University of Cambridge
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