-By contrast, the number-of-calls and basic-block figures
-are derived by counting, not
-sampling. They are completely accurate and will not vary from run to run
-if your program is deterministic.
+By contrast, the number-of-calls and basic-block figures are derived
+by counting, not sampling. They are completely accurate and will not
+vary from run to run if your program is deterministic and single
+threaded. In multi-threaded applications, or single threaded
+applications that link with multi-threaded libraries, the counts are
+only deterministic if the counting function is thread-safe. (Note:
+beware that the mcount counting function in glibc is @emph{not}
+thread-safe). @xref{Implementation, ,Implementation of Profiling}.