Under some circumstances deadlock occurs in the ExpressionCacheImpl.
1)Thread Ta opening a Page Pa. Page Pa does not bound to the current request cycle, so new Instance requested from PageSource and current thread enter to the synchronized _pool.borrowObject(key)
2) At this time thread Tb render page Pb, and for some property evaluates expression binding. So ExpressionCacheImpl's _lock locked by thread Tb. Result of expression evaluation is a reference to the Page Pc instance, which is not bound to the current thread, so new Instance requested from PageSource and current thread tried to enter to the synchronized _pool.borrowObject(key) which is locked by Ta.
3) Thread Ta worked creating new Page Pa Instance, which is not exists in the pool, so a new Page instance should be created. In the proccess of parsing page loader try to evaluate expression binding and try to lock a ExpressionCacheImpl's _lock, which is locked by thread Tb.
4) Whe have a deadlock, so no new page instance and expression could be created and evaluated.
This causes a serios problems while system startup, so we need to restart our app many times, untill it warmed up.