+
+ /*
+ * This function may be called many times to link or re-link the
+ * underlying vnode to the kqueue. If we've already linked the two,
+ * we will have a valid kn_hook_data which ties us to the underlying
+ * device's waitq via a the waitq's prepost table object. However,
+ * devices can abort any select action by calling selthreadclear().
+ * This is OK because the table object will be invalidated by the
+ * driver (through a call to selthreadclear), so any attempt to access
+ * the associated waitq will fail because the table object is invalid.
+ *
+ * Even if we've already registered, we need to pass a pointer
+ * to a reserved link structure. Otherwise, selrecord() will
+ * infer that we're in the second pass of select() and won't
+ * actually do anything!
+ */
+ rsvd = rsvd_arg = waitq_link_reserve(NULL);
+ rlptr = (void *)&rsvd_arg;
+
+ /*
+ * Trick selrecord() into hooking kqueue's wait queue set into the device's
+ * selinfo wait queue.
+ */
+ old_wqs = uth->uu_wqset;
+ uth->uu_wqset = &(knote_get_kq(kn)->kq_wqs);
+ /*
+ * Now these are the laws of VNOP_SELECT, as old and as true as the sky,
+ * And the device that shall keep it may prosper, but the device that shall
+ * break it must receive ENODEV:
+ *
+ * 1. Take a lock to protect against other selects on the same vnode.
+ * 2. Return 1 if data is ready to be read.
+ * 3. Return 0 and call `selrecord` on a handy `selinfo` structure if there
+ * is no data.
+ * 4. Call `selwakeup` when the vnode has an active `selrecord` and data
+ * can be read or written (depending on the seltype).
+ * 5. If there's a `selrecord` and no corresponding `selwakeup`, but the
+ * vnode is going away, call `selthreadclear`.
+ */
+ selres = VNOP_SELECT(vp, knote_get_seltype(kn), 0, rlptr, ctx);
+ uth->uu_wqset = old_wqs;
+
+ /*
+ * Make sure to cleanup the reserved link - this guards against
+ * drivers that may not actually call selrecord().
+ */
+ waitq_link_release(rsvd);
+ if (rsvd != rsvd_arg) {
+ /* The driver / handler called selrecord() */
+ struct waitq *wq;
+ memcpy(&wq, rlptr, sizeof(void *));
+
+ /*
+ * The waitq is part of the selinfo structure managed by the
+ * driver. For certain drivers, we want to hook the knote into
+ * the selinfo structure's si_note field so selwakeup can call
+ * KNOTE.
+ */
+ si = selinfo_from_waitq(wq);
+
+ /*
+ * The waitq_get_prepost_id() function will (potentially)
+ * allocate a prepost table object for the waitq and return
+ * the table object's ID to us. It will also set the
+ * waitq_prepost_id field within the waitq structure.
+ *
+ * We can just overwrite kn_hook_data because it's simply a
+ * table ID used to grab a reference when needed.
+ *
+ * We have a reference on the vnode, so we know that the
+ * device won't go away while we get this ID.
+ */
+ kn->kn_hook_data = waitq_get_prepost_id(wq);
+ } else if (selres == 0) {
+ /*
+ * The device indicated that there's no data to read, but didn't call
+ * `selrecord`. Nothing will be notified of changes to this vnode, so
+ * return an error back to user space, to make it clear that the knote
+ * is not attached.
+ */
+ knote_set_error(kn, ENODEV);
+ }
+
+ vnode_put(vp);
+
+ return selres;
projects / apple / xnu.git / blobdiff