Many dynamic updating systems have been developed that enable a program to be patched while it runs, to fix bugs or add new features. This paper explores techniques for supporting dynamic updates to multi-threaded programs, focusing on the problem of applying an update in a timely fashion while still producing correct behavior. Past work has shown that this tension of safety versus timeliness can be balanced for single-threaded programs. For multi-threaded programs, the task is more difficult because myriad thread interactions complicate understanding the possible program states to which a patch could be applied. Our approach allows the programmer to specify a few program points (e.g., one per thread) at which a patch may be applied, which simplifies reasoning about safety. To improve timeliness, a combination of static analysis and runtime support automatically expands these few points to many more that produce behavior equivalent to the originals. Experiments with thirteen realistic updates to three multi-threaded servers show that we can safely perform a dynamic update within milliseconds when more straightforward alternatives would delay some updates indefinitely
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