[Git][ghc/ghc][wip/T22965] rts: Statically assert alignment of Capability

[Ben Gamari (@bgamari)]

Ben Gamari pushed to branch wip/T22965 at Glasgow Haskell Compiler / GHC


Commits:
d5f765ce by Ben Gamari at 2023-02-13T14:27:02-05:00
rts: Statically assert alignment of Capability

In #22965 we noticed that changes in the size of `Capability` can result
in unsound behavior due to the `align` pragma claiming an alignment
which we don't in practice observe. Avoid this by statically asserting
that the size is a multiple of the alignment.

- - - - -


1 changed file:

- rts/Capability.h


Changes:

=====================================
rts/Capability.h
=====================================
@@ -28,6 +28,14 @@
 
 #include "BeginPrivate.h"
 
+// We never want a Capability to overlap a cache line with
+// anything else, so round it up to a cache line size:
+#if defined(s390x_HOST_ARCH)
+#define CAPABILITY_ALIGNMENT 256
+#elif !defined(mingw32_HOST_OS)
+#define CAPABILITY_ALIGNMENT 64
+#endif
+
 /* N.B. This must be consistent with CapabilityPublic in RtsAPI.h */
 struct Capability_ {
     // State required by the STG virtual machine when running Haskell
@@ -169,14 +177,16 @@ struct Capability_ {
     StgTRecHeader *free_trec_headers;
     uint32_t transaction_tokens;
 } // typedef Capability is defined in RtsAPI.h
-  // We never want a Capability to overlap a cache line with anything
-  // else, so round it up to a cache line size:
-#if defined(s390x_HOST_ARCH)
-  ATTRIBUTE_ALIGNED(256)
-#elif !defined(mingw32_HOST_OS)
-  ATTRIBUTE_ALIGNED(64)
+#if defined(CAPABILITY_ALIGNMENT)
+  ATTRIBUTE_ALIGNED(CAPABILITY_ALIGNMENT)
+#endif
+;
+
+// We allocate arrays of Capabilities therefore we must ensure that the size is
+// a multiple of the claimed alignment
+#if defined(CAPABILITY_ALIGNMENT)
+GHC_STATIC_ASSERT(sizeof(struct Capability_) % CAPABILITY_ALIGNMENT == 0, "Capability size does not match cache size");
 #endif
-  ;
 
 #if defined(THREADED_RTS)
 #define ASSERT_TASK_ID(task) ASSERT(task->id == osThreadId())



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/d5f765ce080f5f6877a32665a7b0a7c443805a84

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/d5f765ce080f5f6877a32665a7b0a7c443805a84
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