kqueue and device driver experience anyone ?

Luigi Rizzo rizzo at iet.unipi.it
Fri Aug 26 15:42:09 UTC 2011

a question for the kqueue experts out there:

I am trying to add kqueue support to a device driver, and am puzzled
on what the .f_event() function may assume.

I see that some of the examples (e.g. bpf, audit_pipe.c)
expect that the function is called with the device lock held
(and even have a LOCK_ASSERT).

Others (if_tap, cam/scsi/scsi_target.c) either do not use the lock
or explicitly acquire it.

As far as i can tell the .f_event() function is called in two places:

- within knote() which in turn (through KNOTE_*() ) is called
  by the driver itself near selrecord() . So it is up to the
  device driver to call it with the device lock held;

- within kqueue_scan(), which instead is called from the upper half
  of the kernel as part of kern_kevent(). Here there seems to be no
  way that the device lock is acquired when .f_event() is called.
  Unless, of course, the knote's on which these .f_event() are
  called are not the same ones attached to devices -- so there is
  a different .f_event() function called ?

So, is there a bug in the kqueue support for bpf.c and audit_pipe.c,
or i am missing something important ?


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