The patch at

makes midi(4) MP-safe. With this patch, my kernel with WITNESS and
INVARIANTS plays my favorite midi tunes just fine, except that the
callout used by the midi sequencer gets delayed under a heavy load. As
the sequencer uses callout in order to wait until the next midi
message is ready to be transmitted, the callout should run at PI_AV so
that a tune will not sound messy. John, have you got any plan to
modify the priority of a callout?

Cameron, as pcm(4) interacts with few of the other subsystems in the
kernel like midi(4), why don't you start making pcm(4) MP-safe?
Multimedia devices should benefit very much from kernel execution in
parallel. The only one thing to note is to not access a device in
parallel to mess it up. We have to add a mutex to a device and share
the mutex between the pcm part and the midi part of the driver. Could
you please let me know when pcm(4) gets MP-safe, and we will see how
we can share device mutexes between pcm(4) and midi(4).


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