LinuxKPI: Add cancel_work() function.

[wulf]

Summary

Cancel a work not waiting for it to finish.

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week

[kib]

sys/compat/linuxkpi/common/src/linux_work.c

374

Should this case be asserted?

[markj]

sys/compat/linuxkpi/common/src/linux_work.c

374

Yes please. I see this is copied from other functions, perhaps the to-state should be WORK_ST_INVALID, and linux_update_state() can assert that pstate[c] is never invalid.

384

We do this non-atomically after the state has transitioned to idle. What if some other thread tries to queue work, so the state goes back from ST_IDLE to ST_TASK, but then queues work before the taskqueue_cancel() call? The state will be left at ST_TASK even though all work is cancelled.