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{ II Reve r s e o r d e r of a c q u i re : b then a Monito r E x it ( b m_sy n c Loc k ) ; Monitor E x it ( a m_sy n c Loc k ) ; } else { II Reve r s e o r d e r of a c q u i re : a t h e n b Monitor E x it ( a m_sy n c Loc k ) ; Monitor E x it ( b m_syn c Loc k ) ; } }
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This approach ensures deadlock free T r a n sfer operations And it doesn't really add any additional overhead, although the reason why it's correct is somewhat subtle It works for our specific example of exactly two Ba n k Ac c o u n t objects, but doesn't scale to all possible cases To support a broader range of scenarios, we can resort to doing a general sort instead
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u s i ng using using using System ; System Co l l e c t i on s Gener i c ; System Thread i n g ; System Runtime Com p i l e r S e rv i c e s ; ICompa r a b l e < T >
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i n t e r n a l s t a t i c c l a s s Mult i Lo c k H e l p e r < T > where T { i n t e r n a l s t a t i c void Enter ( pa rams T [ ] loc k s ) { Array Sort ( lo c k s ) ; I I Now perform t h e wa i t s i n sorted orde r int i = e ; t ry { for ( ; i < loc k s Lengt h ; i++ ) Monitor E nt e r ( loc k s [ i ] ) ; catch II Undo t h e s u c c e s sf u l a c q u i s it i on s for ( int j = i 1 ; j >= e ; j - - ) Mon i t o r E xit ( loc k s [ j ] ) ; t h row ;
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i n t e r n a l s t a t i c void E x it < T > ( pa rams T [ ] loc k s ) { Array Sort ( lo c k s ) j I I E x it t h e loc k s i n reve r s e sorted orde r for ( i nt i loc k s Length - 1 j i > = a j i - - ) Monitor E xit ( loc k s [ i ] ) j
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This code has some disadvantages One clear disadvantage is the performance overhead for doing a sort We also have to do it twice, once for
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E nter and once for E x it, although this could be avoided If the caller passed
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the same locks array to both methods, we could sort it in place in E nt e r and then skip the sort entirely inside of Exit assuming the same array is supplied Another disadvantage is that locks themselves don't always have unique keys associated with them When coding in C++ with C R ITICAL_S E CTIONs, you can sort on the memory address; and with kernel objects, you can use the HAND L E value Both are guaranteed unique and stable But CLR monitors can be any kind of CLR object, so you need to implement ordering at some higher level (hence the restriction in Mu lt i Lo c k H e l pe r < T > above that the generic argument T implements ICompa r a b l e < T We could do this in our Ba n kAc c o u n t example by combining the m_i d and m_sy n c Loc k fields into a single comparable object
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Lock Leveling All of this talk about ordering locks during acquisition brings us to our next technique for avoiding deadlocks: lock leveling This technique is commonly known under several other guises: lock ranking, lock hierarchies, and lock ordering, among others We already stated that if threads always acquire locks in a consistent order, there will be no dead locks, but it may not be obvious why this is true Cycles are required to pro duce a deadlock, and consistent ordering (with no exceptions) eliminates the possibility of cycles Imagine we assign a unique level to each lock in the system (This is stronger than the previous example, where only like locks needed to be disambiguated ) Then, if a thread only waits for locks with a level "less" than the lowest level already currently held, it is enough to guarantee dead lock freedom by construction Strict adherence to the leveling scheme can be statically verified in the best case, and dynamically verified in the worst
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