thread: don't always convert end-atomic to a thread swap

If interrupts are disabled to prevent thread swaps, then don't react
to `end-atomic` by performing a deferred swap. That might happen
with logger callbacks after a GC where a deferred action was
overlooked due to a rare race condition.

racket/src/cs/README.txt

@@ -307,6 +307,66 @@ FFI Differences
    not actually an array of pointers.
 
 
+Threads, Threads, Atomicity, Atomicity, and Atomicity
+-----------------------------------------------------
+
+Racket's thread layer does not use Chez Scheme threads. Chez Scheme
+threads correspond to OS threads. Racket threads are implemented in
+terms of engines at the Rumble layer. At the same time, futures and
+places will use Chez Scheme threads, and so parts of Rumble are meant
+to be thread-safe with respect to Chez Scheme and OS threads. The FFI
+also exposes elements of Chez Scheme / OS threads.
+
+As a result of these layers, there are multiple ways to implement
+atomic regions:
+
+ * For critical sections with respect to Chez Scheme / OS threads, use
+   a mutex.
+
+   For example, the implementation of `eq?` and `eqv?`-based hash
+   tables uses mutex to guard hash tables, so they can be accessed
+   concurrently from futures. In contrast, `equal?`-based hash table
+   operations are not atomic from the Racket perspective, so they
+   can't be locked by a mutex; they use Racket-thread locks, instead.
+   The "rumble/lock.ss" layer skips the `eq?`/`eqv?`-table mutex when
+   threads are not enabled at the Chez Scheme level.
+
+ * For critical sections at the Racket level, there are multiple
+   possibilities:
+
+     - The Racket "thread" layer provides `start-atomic` and
+       `end-atomic` to prevent Racket-thread swaps.
+
+       These are the same opertations as provided by
+       `ffi/unsafe/atomic`.
+
+     - Disabling Chez Scheme interrupts will also disable Racket
+       thread swaps, since a thread swap via engines depends on a
+       timer interrupt --- unless something explicitly blocks via the
+       Racket thread scheduler, such as with `(sleep)`.
+
+       Disable interrupts for atomicity only at the Rumble level where
+       no Racket-level callbacks are not involved. Also, beware that
+       disabling interrupts will prevent GC interrupts.
+
+       The Racket "thread" layer provides `start-atomic/no-interrupts`
+       and `end-atomic/no-interrupts` for both declaing atomicity at
+       the Racket level and turning off Chez Scheme interrupts. The
+       combination is useful for implementing functionality that might
+       be called in response to a GC and might also be called by
+       normal (non-atomic) code; the implementation of logging at the
+       "io" layer might be the only use case.
+
+     - The implementation of engines and continuations uses its own
+       flag to protect regions where an engine timeout should not
+       happen, such as when the metacontinuation is being manipulated.
+       That flag is managed by `start-uninterrupted` and
+       `end-uninterrupted` in "rumble/interrupt.ss".
+
+       It may be tempting to use that flag for other purposes, as a
+       cheap way to disable thread swaps. For now, don't do that.
+
+
 Status and Thoughts on Various Racket Subsystems
 ------------------------------------------------
 
@@ -316,10 +376,6 @@ Status and Thoughts on Various Racket Subsystems
    needed often in a typical program, and the overhead appears to be
    light when it is needed.
 
- * Racket's delimited continuations, continuation marks, threads, and
-   events are mostly in place (see "rumble/control.ss",
-   "rumble/engine.ss", and the source for "thread.rktl").
-
  * The "rktio" library fills the gap between Racket and Chez Scheme's
    native I/O. The "rktio" library provides a minimal, non-blocking,
    non-GCed interface to OS-specific functionality. Its' compiled to a

racket/src/cs/rumble.sls

@@ -45,6 +45,7 @@
 
           make-engine
           engine-block
+          engine-timeout
           engine-return
           current-engine-state  ; not exported to Racket
           set-ctl-c-handler! ; not exported to Racket

racket/src/cs/rumble/engine.ss

@@ -92,6 +92,17 @@
             (engine-state-thread-cell-values es)
             (engine-state-init-break-enabled-cell es)))))))))
 
+(define (engine-timeout)
+  (let ([can-block? (fx= 1 (disable-interrupts))])
+    (enable-interrupts)
+    (cond
+     [can-block?
+      (engine-block)]
+     [else
+      ;; Cause the timer to fire as soon as possible (i.e., as soon
+      ;; as interrupts are enabled)
+      (set-timer 1)])))
+
 (define (engine-return . args)
   (assert-not-in-uninterrupted)
   (timer-interrupt-handler void)

racket/src/cs/rumble/interrupt.ss

@@ -2,7 +2,9 @@
 ;; Enabling uninterrupted mode defers a timer callback
 ;; until leaving uninterrupted mode. This is the same
 ;; as disabling and enabling interrupts at the Chez
-;; level, but cheaper and more limited.
+;; level, but cheaper and more limited. Uninterrupted
+;; mode should be used only by the implementation of
+;; engines and control in "engine.ss" and "control.ss".
 
 (define-virtual-register current-in-uninterrupted #f)
 (define-virtual-register pending-interrupt-callback #f)

racket/src/cs/rumble/memory.ss

@@ -27,7 +27,10 @@
 (define collect-generation-radix-mask (sub1 (bitwise-arithmetic-shift 1 log-collect-generation-radix)))
 (define allocated-after-major (* 32 1024 1024))
 
-;; Called in any thread with all other threads paused
+;; Called in any thread with all other threads paused. The Racket
+;; thread scheduler may be in atomic mode. In fact, the engine
+;; and control layer may be in uninterrupted mode, so don't
+;; do anything that might use "control.ss" (especially in logging).
 (define (collect/report g)
   (let ([this-counter (if g (bitwise-arithmetic-shift-left 1 (* log-collect-generation-radix g)) gc-counter)]
         [pre-allocated (bytes-allocated)]

racket/src/cs/thread.sls

@@ -11,6 +11,7 @@
                   ;; These are extracted via `#%linklet`:
                   [make-engine rumble:make-engine]
                   [engine-block rumble:engine-block]
+                  [engine-timeout rumble:engine-timeout]
                   [engine-return rumble:engine-return]
                   [current-engine-state rumble:current-engine-state]
                   [make-condition rumble:make-condition]
@@ -64,6 +65,7 @@
        (hash
         'make-engine rumble:make-engine
         'engine-block rumble:engine-block
+        'engine-timeout rumble:engine-timeout
         'engine-return rumble:engine-return
         'current-engine-state (lambda (v) (rumble:current-engine-state v))
         'set-ctl-c-handler! rumble:set-ctl-c-handler!

racket/src/io/host/thread.rkt

@@ -3,7 +3,7 @@
 
 (provide atomically
          non-atomically
-         atomically/no-interrupts
+         atomically/no-interrupts/no-wind
          assert-atomic
          check-current-custodian)
 
@@ -70,8 +70,11 @@
       (let () e ...)
       (start-atomic))))
 
-;; Cannot be exited with `non-atomically`:
-(define-syntax-rule (atomically/no-interrupts e ...)
+;; Disables host interrupts, but the "no wind" part is
+;; an unforced constraint: don't use anything related
+;; to `dynamic-wind`, continuations, or continuation marks.
+;; Cannot be exited with `non-atomically`.
+(define-syntax-rule (atomically/no-interrupts/no-wind e ...)
   (begin
     (start-atomic/no-interrupts)
     (begin0

racket/src/io/logger/main.rkt

@@ -101,7 +101,7 @@
   (check who #:or-false symbol? topic)
   (check who string? message)
   (define msg #f)
-  (atomically/no-interrupts
+  (atomically/no-interrupts/no-wind
    (when ((logger-max-wanted-level logger) . level>=? . level)
      (let loop ([logger logger])
        (for ([r (in-list (logger-receivers logger))])

racket/src/io/logger/receiver.rkt

@@ -29,7 +29,7 @@
     ;; called in atomic mode and possibly in host interrupt handler,
     ;; so anything we touch here should only be modified with
     ;; interrupts disabled
-    (atomically/no-interrupts
+    (atomically/no-interrupts/no-wind
      (define b (queue-remove! (queue-log-receiver-waiters lr)))
      (cond
        [b
@@ -41,19 +41,20 @@
   #:property
   prop:evt
   (poller (lambda (lr ctx)
-            (define msg (atomically/no-interrupts (queue-remove! (queue-log-receiver-msgs lr))))
+            (define msg (atomically/no-interrupts/no-wind (queue-remove! (queue-log-receiver-msgs lr))))
             (cond
               [msg
                (values (list msg) #f)]
               [else
                (define b (box (poll-ctx-select-proc ctx)))
-               (define n (atomically/no-interrupts (queue-add! (queue-log-receiver-waiters lr) b)))
+               (define n (atomically/no-interrupts/no-wind (queue-add! (queue-log-receiver-waiters lr) b)))
                (values #f (control-state-evt
                            (wrap-evt async-evt (lambda (e) (unbox b)))
-                           (lambda () (atomically/no-interrupts (queue-remove-node! (queue-log-receiver-waiters lr) n)))
+                           (lambda () (atomically/no-interrupts/no-wind
+                                       (queue-remove-node! (queue-log-receiver-waiters lr) n)))
                            void
                            (lambda ()
-                             (atomically/no-interrupts
+                             (atomically/no-interrupts/no-wind
                               (define msg (queue-remove! (queue-log-receiver-msgs lr)))
                               (cond
                                 [msg
@@ -106,7 +107,7 @@
 ;; ----------------------------------------
 
 (define (add-log-receiver! logger lr)
-  (atomically/no-interrupts
+  (atomically/no-interrupts/no-wind
    ;; Add receiver to the logger's list, purning empty boxes
    ;; every time the list length doubles (roughly):
    (cond

racket/src/io/logger/wanted.rkt

@@ -9,7 +9,7 @@
          logger-all-levels)
 
 (define (logger-wanted-level logger topic)
-  (atomically/no-interrupts
+  (atomically/no-interrupts/no-wind
    (cond
      [(not topic) (logger-max-wanted-level logger)]
      [else
@@ -30,7 +30,7 @@
          (logger-wanted-level logger topic)])])))
                          
 (define (logger-max-wanted-level logger)
-  (atomically/no-interrupts
+  (atomically/no-interrupts/no-wind
    (cond
      [((logger-local-level-timestamp logger) . >= . (unbox (logger-root-level-timestamp-box logger)))
       ;; Ccahed value is up-to-date

racket/src/thread/atomic.rkt

@@ -45,6 +45,10 @@
      (cb)]
     [(negative? n) (internal-error "not in atomic mode to end")]
     [else
+     ;; There's a small chance that `end-atomic-callback`
+     ;; was set by the scheduler after the check and
+     ;; before we exit atomic mode. Make sure that rare
+     ;; event is ok.
      (current-atomic n)]))
 
 (define (start-atomic/no-interrupts)

racket/src/thread/bootstrap.rkt

@@ -83,6 +83,9 @@
 (define (engine-block)
   (thread-suspend (current-thread)))
 
+(define (engine-timeout)
+  (void))
+
 (define ctl-c-handler #f)
 
 (define (set-ctl-c-handler! proc)
@@ -144,6 +147,7 @@
                  (hash 
                   'make-engine make-engine
                   'engine-block engine-block
+                  'engine-timeout engine-timeout
                   'engine-return (lambda args
                                    (error "engine-return: not ready"))
                   'current-process-milliseconds current-process-milliseconds

racket/src/thread/engine.rkt

@@ -29,6 +29,7 @@
 (bounce #%engine
         make-engine
         engine-block
+        engine-timeout
         engine-return
         current-engine-state
         current-process-milliseconds

racket/src/thread/schedule.rkt

@@ -88,8 +88,10 @@
                (set-thread-engine! t e))
              (select-thread!)]
             [else
-             ;; Swap out when the atomic region ends:
-             (set-end-atomic-callback! engine-block)
+             ;; Swap out when the atomic region ends and at a point
+             ;; where host-system interrupts are not disabled (i.e.,
+             ;; don't use `engine-block` instead of `engine-timeout`):
+             (set-end-atomic-callback! engine-timeout)
              (loop e)])))))))
 
 (define (maybe-done callbacks)