Hi Ian,

To recap my understanding of the mechanisms at work (glossing over the
queue handling and condvars involved etc), the bhyve block_if
infrastructure registers a callback for SIGCONT with the mevent
subsystem, which is a kevent/kqueue thing which delivers events to the
main thread (mevent_dispatch is the last thing in main()) it also sets

That's correct. The intent was to have the signal delivered via the kevent callback rather than standard signal delivery.

When a disk controller device model wants to
cancel a block request (e.g. in ahci_port_stop) it calls
blockif_cancel which sends a SIGCONT to the blkio thread which has
claimed the request, notionally to kick it out of whatever blocking
system call it is in and cause it to return an error to the device

 Yep, that's correct.

The main thing I do not follow is whether or not the blkio thread is
actually interrupted at all when the signal has been configured to be
delivered via the kevent/kqueue mechanisms to a 3rd unrelated thread.

 It is interrupted on FreeBSD.

I've dug around in the FreeBSD kevent and signal man pages but I
cannot find any part which describes anything of the semantics which
bhyve seems to be relying on (which seems to be that the system call
in the target thread will return EINTR at some point before the thread
which is "handling" the signal via kevent/kqueue sees that event).

Have I missed something here or is bhyve relying on some subtle
underlying semantics?

I didn't think it too FreeBSD-specific - if a thread is blocked in a system call, sending a signal should force it to exit on most Unices.

I have a secondary concern which is what happens if the IO thread is
on its way to making a blocking system call in blockif_proc but has
not actually done so when the signal is delivered. It seems like it
would simply carry on and make the blocking call with perhaps
unexpected consequences (i/o getting wedged, perhaps only until a
second reset attempt). I've not actually seen this happening though
and there's a chance I'm simply over thinking things after staring at
them for so long!

I believe this case is handled - I discussed this at length with Tycho when the code was committed a while back.

 Tycho - any thoughts ?


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