[XRay][docs] Define requirements on installed log handlers.

deanberris · deanberris · commit 6eec7d4158fd · 2016-11-16T02:18:23.000Z
Summary: We update the documentation to define what the requirements are for the provided XRay log handler. This is to make it clear that the function pointer provided must do internal synchronisation and that there are no guarantees provided by XRay on when the function shall be invoked once it has been installed as a log handler. Reviewers: rSerge, rengolin Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D26651 llvm-svn: 287073
diff --git a/llvm/docs/XRay.rst b/llvm/docs/XRay.rst
@@ -162,6 +162,19 @@ functions we list below:
 - ``__xray_unpatch()``: Unpatch the instrumentation points defined in the
   binary.
 
+There are some requirements on the logging handler to be installed for the
+thread-safety of operations to be performed by the XRay runtime library:
+
+- The function should be thread-safe, as multiple threads may be invoking the
+  function at the same time. If the logging function needs to do
+  synchronisation, it must do so internally as XRay does not provide any
+  synchronisation guarantees outside from the atomicity of updates to the
+  pointer.
+- The pointer provided to ``__xray_set_handler(...)`` must be live even after
+  calls to ``__xray_remove_handler()`` and ``__xray_unpatch()`` have succeeded.
+  XRay cannot guarantee that all threads that have ever gotten a copy of the
+  pointer will not invoke the function.
+
 
 Trace Analysis Tools
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