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authorBjörn Gustavsson <[email protected]>2014-02-28 06:34:43 +0100
committerBjörn Gustavsson <[email protected]>2014-02-28 08:04:04 +0100
commit63a653050b5e6ca28cde1e8616c840abafe71100 (patch)
tree07d3ab04eeea4ef290d739a9b8f4d9758fcbeeab /.gitattributes
parent81c7db0e247a6ee1b7a5c2764aa9575d7f6fb88a (diff)
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Delay patching of closures to eliminate a race condition
The loader of native code tries to avoid race condition by bringing down the system to a single scheduler. Unfortunately, that will not completely avoid race conditions beacuse other processes may still run while the code is being loaded. Therefore, we must still make sure that native code has been fully fixed up before it can be executed. Under unlucky circumstances, it was possible for the global name register process to call a fun in native code before the native code for the fun had been fully fixed up. The run-time system would crash when running the not-fully-fixed-up code. Here is a way to make the bug 100% reproducible. First add this function to the 'global' module: silly_looper(List) -> lists:foreach(fun(E) -> put(any_atom, E) end, List), silly_looper(List). Then insert the following code at the beginning of the init/1 function in the 'global' module: spawn_link(fun() -> silly_looper(lists:seq(1, 100)) end), The run-time system will crash during start up when the native code for the 'global' module is being loaded. What will happen is that lists:foreach/2 (which runs in native code) will call the native code for the fun in silly_looper/1 before the reference to 'any_atom' has been changed from a 0 to the proper tagged atom value. The put/2 BIF will exit when attempting to calculate the hash value for the illegal value 0 (it is not properly tagged). To eliminate this race condition, we must delay patching the native code addresses for a fun into the fun entry until the native code has been fully fixed up. We will use the new BIFs introduced in the previous commit. While we are at it, we will also make sure that we erase any temporary variables in the process dictionary used by the hipe_unified_loader module.
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