X-Git-Url: http://repo.macrolet.net/gitweb/?a=blobdiff_plain;f=src%2Fcode%2Fsysmacs.lisp;h=60f18d751abd861436e9f5bd1eea2a7696548106;hb=ee222567ee95eaac8f6f4c877242dd116bfb8337;hp=bcb82555a2fccde30c1a3ea614784a1f82bde9b5;hpb=7beac8a1fb3cfdeda0e168eb597a701951032270;p=sbcl.git

diff --git a/src/code/sysmacs.lisp b/src/code/sysmacs.lisp
index bcb8255..60f18d7 100644
--- a/src/code/sysmacs.lisp
+++ b/src/code/sysmacs.lisp
@@ -32,20 +32,46 @@
 
 (defmacro without-gcing (&body body)
   #!+sb-doc
-  "Executes the forms in the body without doing a garbage
-collection. It inhibits both automatically and explicitly triggered
-gcs. Finally, upon leaving the BODY if gc is not inhibited it runs the
-pending gc. Similarly, if gc is triggered in another thread then it
-waits until gc is enabled in this thread."
-  `(unwind-protect
-    (let ((*gc-inhibit* t))
-      ,@body)
-    ;; the test is racy, but it can err only on the overeager side
-    (when (and (not *gc-inhibit*)
-               (or #!+sb-thread *stop-for-gc-pending*
-                   *gc-pending*))
-      (sb!unix::receive-pending-interrupt))))
-
+  "Executes the forms in the body without doing a garbage collection. It
+inhibits both automatically and explicitly triggered collections. Finally,
+upon leaving the BODY if gc is not inhibited it runs the pending gc.
+Similarly, if gc is triggered in another thread then it waits until gc is
+enabled in this thread.
+
+Implies SB-SYS:WITHOUT-INTERRUPTS for BODY, and causes any nested
+SB-SYS:WITH-INTERRUPTS to signal a warning during execution of the BODY.
+
+Should be used with great care, and not at all in multithreaded application
+code: Any locks that are ever acquired while GC is inhibited need to be always
+held with GC inhibited to prevent deadlocks: if T1 holds the lock and is
+stopped for GC while T2 is waiting for the lock inside WITHOUT-GCING the
+system will be deadlocked. Since SBCL does not currently document its internal
+locks, application code can never be certain that this invariant is
+maintained."
+  (with-unique-names (without-gcing-body)
+    `(flet ((,without-gcing-body ()
+              ,@body))
+       (if *gc-inhibit*
+           (,without-gcing-body)
+           (without-interrupts
+             ;; We need to disable interrupts before disabling GC, so that
+             ;; signal handlers using locks don't accidentally try to grab
+             ;; them with GC inhibited.
+             ;;
+             ;; It would be nice to implement this with just a single UWP, but
+             ;; unfortunately it seems that it cannot be done: the naive
+             ;; solution of binding both *INTERRUPTS-ENABLED* and
+             ;; *GC-INHIBIT*, and checking for both pending GC and interrupts
+             ;; in the cleanup breaks if we have a GC pending, but no
+             ;; interrupts, and we receive an asynch unwind while checking for
+             ;; the pending GC: we unwind before handling the pending GC, and
+             ;; will be left running with further GCs blocked due to the GC
+             ;; pending flag.
+             (unwind-protect
+                  (let ((*gc-inhibit* t))
+                    (,without-gcing-body))
+               (when (or *gc-pending* #!+sb-thread *stop-for-gc-pending*)
+                 (sb!unix::receive-pending-interrupt))))))))
 
 ;;; EOF-OR-LOSE is a useful macro that handles EOF.
 (defmacro eof-or-lose (stream eof-error-p eof-value)
@@ -119,6 +145,9 @@ waits until gc is enabled in this thread."
 ;;; This macro sets up some local vars for use by the
 ;;; FAST-READ-CHAR macro within the enclosed lexical scope. The stream
 ;;; is assumed to be a ANSI-STREAM.
+;;;
+;;; KLUDGE: Some functions (e.g. ANSI-STREAM-READ-LINE) use these variables
+;;; directly, instead of indirecting through FAST-READ-CHAR.
 (defmacro prepare-for-fast-read-char (stream &body forms)
   `(let* ((%frc-stream% ,stream)
           (%frc-method% (ansi-stream-in %frc-stream%))
@@ -134,17 +163,22 @@ waits until gc is enabled in this thread."
   `(setf (ansi-stream-in-index %frc-stream%) %frc-index%))
 
 ;;; a macro with the same calling convention as READ-CHAR, to be used
-;;; within the scope of a PREPARE-FOR-FAST-READ-CHAR
+;;; within the scope of a PREPARE-FOR-FAST-READ-CHAR.
 (defmacro fast-read-char (&optional (eof-error-p t) (eof-value ()))
   `(cond
-    ((not %frc-buffer%)
-     (funcall %frc-method% %frc-stream% ,eof-error-p ,eof-value))
-    ((= %frc-index% +ansi-stream-in-buffer-length+)
-     (prog1 (fast-read-char-refill %frc-stream% ,eof-error-p ,eof-value)
-            (setq %frc-index% (ansi-stream-in-index %frc-stream%))))
-    (t
-     (prog1 (aref %frc-buffer% %frc-index%)
-            (incf %frc-index%)))))
+     ((not %frc-buffer%)
+      (funcall %frc-method% %frc-stream% ,eof-error-p ,eof-value))
+     ((= %frc-index% +ansi-stream-in-buffer-length+)
+      (multiple-value-bind (eof-p index-or-value)
+          (fast-read-char-refill %frc-stream% ,eof-error-p ,eof-value)
+        (if eof-p
+            index-or-value
+            (progn
+              (setq %frc-index% (1+ index-or-value))
+              (aref %frc-buffer% index-or-value)))))
+     (t
+      (prog1 (aref %frc-buffer% %frc-index%)
+        (incf %frc-index%)))))
 
 ;;;; And these for the fasloader...