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Hi Sergei, I'm afraid I couldn't find easy fix for this deadlock. The question is if it is worth to spend time for more complex (and possibly less stable) solution. Details below. Locking profile --------------- INSTALL PLUGIN: LOCK_plugin -> LOCK_system_variables_hash (w) SHOW VARIABLES: LOCK_system_variables_hash (r) -> LOCK_global_system_variables change_user(): LOCK_global_system_variables -> LOCK_plugin INSTALL PLUGIN -------------- Needs LOCK_plugin to register new plugin. Needs LOCK_system_variables_hash to register plugin variables. Doesn't seem to need both locks at the same time. Similar lock order is used in a few places. SHOW VARIABLES -------------- Needs LOCK_system_variables_hash to iterate system_variable_hash. Needs LOCK_global_system_variables to read variable value. Does seem to need both locks at the same time. This lock order is not used in other places. change_user() ------------- Needs LOCK_global_system_variables to read global_system_variable.table_plugin. Needs LOCK_plugin to my_plugin_lock(table_plugin). Does seem to need both locks at the same time. Similar lock order is used in a few places. "SHOW VARIABLES" looks correct. So I have only two ideas: - Fix INSTALL PLUGIN so it releases LOCK_plugin before registering variables. Sounds like the best solution, but there are a few more things to fix, e.g. UNINTALL PLUGIN. - Store default storage engine as a string, not Sys_var_plugin. Thanks, Sergey