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Switch from timer-expired /messages/ to /assertions/ at ground level. 

Previously, the timer driver caused the background thread to call
send-ground-message to indicate that a timer had expired. However,
this can lead to a race! In cases where a timer expires very soon, the
channel-put of the set-timer instruction leads shortly thereafter to a
send-ground-message which then races the establishment of the
metalevel-1 subscription to the timer-expired events that are coming
from the background thread.

The race cannot occur in the sequential implementation because the
network makes sure to enqueue the transition actions resulting from
the set-timer message delivery ahead of any enqueueing of the
timer-expired ground message, so that by the time the ground message
is processed, the relevant subscription always exists.

In a looser implementation, however, this level of synchronised
activity may not exist, and the ground message may overtake the
subscription establishment.

Therefore, I've changed the driver to instead use ground /assertions/
to signal expired timers. Upon processing of such an assertion, the
driver cleans it up. This is very similar to hardware interrupts,
where the driver has to "clear the interrupt" in order to let the
system continue properly.
This commit is contained in:
Tony Garnock-Jones 2016-01-21 17:38:12 -05:00
parent 5e6d72b991
commit e1c5fd4ac1
2 changed files with 54 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

51
prospect/drivers/timer.rkt
View File

@ -19,15 +19,53 @@
(struct set-timer (label msecs kind) #:prefab)
(struct timer-expired (label msecs) #:prefab)
(define expiry-projection (compile-projection (at-meta (?! (timer-expired ? ?)))))
(define (spawn-timer-driver)
(define control-ch (make-channel))
(thread (lambda () (timer-driver-thread-main control-ch)))
(define (timer-driver e count)
(match e
[(message (at-meta (and expiry (timer-expired _ _))))
(transition (- count 1)
(list (message expiry)
(when (= count 1) (unsub (timer-expired ? ?) #:meta-level 1))))]
[(patch added _removed)
;; Previously, this driver caused the background thread to call
;; send-ground-message to indicate that a timer had expired.
;; However, this can lead to a race! In cases where a timer
;; expires very soon, the channel-put of the set-timer
;; instruction leads shortly thereafter to a
;; send-ground-message which then races the establishment of
;; the metalevel-1 subscription to the timer-expired events
;; that are coming from the background thread.
;;
;; The race cannot occur in the sequential implementation
;; because the network makes sure to enqueue the transition
;; actions resulting from the set-timer message delivery ahead
;; of any enqueueing of the timer-expired ground message, so
;; that by the time the ground message is processed, the
;; relevant subscription always exists.
;;
;; In a looser implementation, however, this level of
;; synchronised activity may not exist, and the ground message
;; may overtake the subscription establishment.
;;
;; Therefore, I've changed the driver to instead use ground
;; /assertions/ to signal expired timers. Upon processing of
;; such an assertion, the driver cleans it up. This is very
;; similar to hardware interrupts, where the driver has to
;; "clear the interrupt" in order to let the system continue
;; properly.
(define-values (new-count actions-rev interrupt-clearing-patch)
(for/fold [(count count)
(actions-rev '())
(interrupt-clearing-patch empty-patch)]
[(expiry (matcher-project/set/single added expiry-projection))]
(values (- count 1)
(cons (message expiry) actions-rev)
(patch-seq interrupt-clearing-patch
(retract expiry)))))
(send-ground-patch interrupt-clearing-patch)
(transition new-count
(cons (reverse actions-rev)
(when (zero? new-count) (unsub (timer-expired ? ?) #:meta-level 1))))]
[(message (and instruction (set-timer _ _ _)))
(channel-put control-ch instruction)
(transition (+ count 1)
@ -45,7 +83,10 @@
[#f never-evt]
[t (handle-evt (timer-evt (pending-timer-deadline t))
(lambda (now)
(for-each send-ground-message (fire-timers! heap now))
(send-ground-patch
(for/fold [(interrupt-asserting-patch empty-patch)]
[(expiry (fire-timers! heap now))]
(patch-seq interrupt-asserting-patch (assert expiry))))
(loop)))])
(handle-evt control-ch
(match-lambda

8
prospect/ground.rkt
View File

@ -12,6 +12,7 @@
(provide (struct-out external-event)
send-ground-message
send-ground-patch
run-ground)
;;---------------------------------------------------------------------------
@ -27,6 +28,13 @@
(define (send-ground-message body)
(async-channel-put (current-ground-event-async-channel) (message body)))
;; Patch -> Void
;; Injects a patch into the ground-VM metalevel. It will appear to be
;; asserted by the environment in general. The obligation is on the caller
;; to ensure that patches do not interfere between drivers.
(define (send-ground-patch p)
(async-channel-put (current-ground-event-async-channel) p))
;;---------------------------------------------------------------------------
;; Communication via RacketEvents