[Git][ghc/ghc][master] 2 commits: rts/linker: Don't unload code when profiling is enabled

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
8b2513e8 by Ben Gamari at 2024-03-11T01:20:03-04:00
rts/linker: Don't unload code when profiling is enabled

The heap census may contain references (e.g. `Counter.identity`) to
static data which must be available when the census is reported at the
end of execution.

Fixes #24512.

- - - - -
7810b4c3 by Ben Gamari at 2024-03-11T01:20:03-04:00
rts/linker: Don't unload native objects when dlinfo isn't available

To do so is unsafe as we have no way of identifying references to
symbols provided by the object.

Fixes #24513. Fixes #23993.

- - - - -


5 changed files:

- rts/CheckUnload.c
- rts/Linker.c
- rts/LinkerInternals.h
- rts/linker/Elf.c
- testsuite/tests/rts/linker/unload_multiple_objs/linker_unload_multiple_objs.c


Changes:

=====================================
rts/CheckUnload.c
=====================================
@@ -165,6 +165,18 @@ ObjectCode *loaded_objects;
 // map static closures to their ObjectCode.
 static OCSectionIndices *global_s_indices = NULL;
 
+// Is it safe for us to unload code?
+static bool safeToUnload(void)
+{
+    if (RtsFlags.ProfFlags.doHeapProfile != NO_HEAP_PROFILING) {
+        // We mustn't unload anything as the heap census may contain
+        // references into static data (e.g. cost centre names).
+        // See #24512.
+        return false;
+    }
+    return true;
+}
+
 static OCSectionIndices *createOCSectionIndices(void)
 {
     // TODO (osa): Maybe initialize as empty (without allocation) and allocate
@@ -457,6 +469,8 @@ void checkUnload(void)
 {
     if (global_s_indices == NULL) {
         return;
+    } else if (!safeToUnload()) {
+        return;
     }
 
     // At this point we've marked all dynamically loaded static objects
@@ -478,8 +492,6 @@ void checkUnload(void)
         next = oc->next;
         ASSERT(oc->status == OBJECT_UNLOADED);
 
-        removeOCSectionIndices(s_indices, oc);
-
         // Symbols should be removed by unloadObj_.
         // NB (osa): If this assertion doesn't hold then freeObjectCode below
         // will corrupt symhash as keys of that table live in ObjectCodes. If
@@ -487,8 +499,17 @@ void checkUnload(void)
         // RTS) then it's probably because this assertion did not hold.
         ASSERT(oc->symbols == NULL);
 
-        freeObjectCode(oc);
-        n_unloaded_objects -= 1;
+        if (oc->unloadable) {
+            removeOCSectionIndices(s_indices, oc);
+            freeObjectCode(oc);
+            n_unloaded_objects -= 1;
+        } else {
+            // If we don't have enough information to
+            // accurately determine the reachability of
+            // the object then hold onto it.
+            oc->next = objects;
+            objects = oc;
+        }
     }
 
     old_objects = NULL;


=====================================
rts/Linker.c
=====================================
@@ -1385,6 +1385,8 @@ mkOc( ObjectType type, pathchar *path, char *image, int imageSize,
    oc->prev              = NULL;
    oc->next_loaded_object = NULL;
    oc->mark              = object_code_mark_bit;
+   /* this will get cleared by the caller if object is not safely unloadable */
+   oc->unloadable        = true;
    oc->dependencies      = allocHashSet();
 
 #if defined(NEED_M32)


=====================================
rts/LinkerInternals.h
=====================================
@@ -313,8 +313,14 @@ struct _ObjectCode {
     struct _ObjectCode *next_loaded_object;
 
     // Mark bit
+    // N.B. This is a full word as we CAS it.
     StgWord mark;
 
+    // Can this object be safely unloaded? Not true for
+    // dynamic objects when dlinfo is not available as
+    // we cannot determine liveness.
+    bool unloadable;
+
     // Set of dependencies (ObjectCode*) of the object file. Traverse
     // dependencies using `iterHashTable`.
     //
@@ -376,7 +382,9 @@ struct _ObjectCode {
     /* handle returned from dlopen */
     void *dlopen_handle;
 
-    /* virtual memory ranges of loaded code */
+    /* virtual memory ranges of loaded code. NULL if no range information is
+     * available (e.g. if dlinfo is unavailable on the current platform).
+     */
     NativeCodeRange *nc_ranges;
 };
 


=====================================
rts/linker/Elf.c
=====================================
@@ -2186,6 +2186,10 @@ void * loadNativeObj_ELF (pathchar *path, char **errmsg)
      copyErrmsg(errmsg, "dl_iterate_phdr failed to find obj");
      goto dl_iterate_phdr_fail;
    }
+   nc->unloadable = true;
+#else
+   nc->nc_ranges = NULL;
+   nc->unloadable = false;
 #endif /* defined (HAVE_DLINFO) */
 
    insertOCSectionIndices(nc);


=====================================
testsuite/tests/rts/linker/unload_multiple_objs/linker_unload_multiple_objs.c
=====================================
@@ -64,7 +64,7 @@ void check_object_freed(char *obj_path) {
     OStatus st;
     st = getObjectLoadStatus(toPathchar(obj_path));
     if (st != OBJECT_NOT_LOADED) {
-        errorBelch("object %s status != OBJECT_NOT_LOADED", obj_path);
+        errorBelch("object %s status != OBJECT_NOT_LOADED, is %d instead", obj_path, st);
         exit(1);
     }
 }



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/dc207d06e9a42bfb531cc0d24271547557b1c488...7810b4c37c7d03772215102a8db5e84d29fe2221

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/dc207d06e9a42bfb531cc0d24271547557b1c488...7810b4c37c7d03772215102a8db5e84d29fe2221
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