Merge pull request #1239 from sorear/sc-svadu

Explicitly allow side effects for a failed SC

src/a-st-ext.adoc

@@ -62,10 +62,11 @@ if the reservation is still valid and the reservation set contains the
 bytes being written. If the SC.W succeeds, the instruction writes the
 word in _rs2_ to memory, and it writes zero to _rd_. If the SC.W fails,
 the instruction does not write to memory, and it writes a nonzero value
-to _rd_. Regardless of success or failure, executing an SC.W instruction
-invalidates any reservation held by this hart. LR.D and SC.D act
-analogously on doublewords and are only available on RV64. For RV64,
-LR.W and SC.W sign-extend the value placed in _rd_.
+to _rd_.  For the purposes of memory protection, a failed SC.W may be
+treated like a store.  Regardless of success or failure, executing an
+SC.W instruction invalidates any reservation held by this hart. LR.D and
+SC.D act analogously on doublewords and are only available on RV64. For
+RV64, LR.W and SC.W sign-extend the value placed in _rd_.
 
 [NOTE]
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