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should be measured and explicitly decided based on the quality bar for the product at the time the bug is found Remember that as higher degrees of concurrency are used in the hardware, the proba bility of these bugs resurfacing becomes higher A race condition that reproduces only once in a while on high-end machines in 2008 could begin happening routinely on middle-of-the-line machines just a couple years later If you decided in 2008 to ship as is, you may pay for that decision in 201 0 when support costs demand that you supply a costly servicing fix
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14 Every lock acquisition might throw an exception Be prepared for it Most locks lazily allocate a kernel event object if a lock acquisition encounters contention, including CLR monitors This allocation can fail during low resource conditions, causing OOMs originating from the entrance to the lock (A typical nonblocking spin lock cannot fail with OOM, which allows it to be used in some resource constrained scenarios where normal locks might be off-limits) Thread interrup tions can lead to Th re a d l n t e r r u pt e d E x c e pt i o n s And SQL Server can perform deadlock detection and even break those deadlocks by throwing a System R u nt ime I n t e ro p S e rv i c e s COM E x c e pt io n Often there isn't much that can b e done i n response t o such an exception, except for letting it unwind the stack This unwind should be done cleanly so that the process doesn't deadlock or crash Reliability and security sensitive code that must deal with failure robustly should consider this possible point of failure and may need to take special action like reverting partially made updates 1 5 Lock leveling should be used to avoid deadlocks Lock leveling is a scheme in which a relative number is assigned to all locks, and a strict ordering among them is enforced This disci pline guarantees deadlock freedom, as was described in 1 1 , Concurrency Hazards Without using something such as lock leveling, libraries are usually subject to dynamic composition and reentrancy induced
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deadlocks, causing users trying t o write even moderately reliable code a lot of frustration This frustration only becomes worse as library usage is woven throughout a highly concurrent application All that said, there are two problems that will surely get in the way of adopting lock leveling today First, there is no standard leveled lock type in the NET Framework today While 11 contains a sample for one, most library devel opers will not start adopting lock leveling in any serious way without an official NET base class and associated guidelines It's also difficult to be successful building libraries that use lock leveling without good tooling support That last statement ties into the second problem: lock leveling is a very onerous discipline The CLR uses it internally for the parts of the system that are relatively closed, but lock leveling doesn't apply so well when dynamic composition is used Levels are represented using numbering schemes that are arbitrarily chosen on a per assembly basis You can develop schemes to extend levels across assemblies, and possibly even cook up some native interoperability story, but these are all features that would have to be built on top of the base lock leveling scheme Again, without standard library support, having to build all this yourself as a library developer is often a nonstarter Lock leveling is one of the more promising techniques that we have for avoiding deadlocks An alternative to lock leveling is to use only nonrecursive locks and closed lock regions This is a good prac tice to follow wherever possible 1 6 Restore sensitive invariants in the face of an exception before the first pass executes up the stack This is in part a security concern as well as a reliability concern The CLR exception model is the two-pass model inherited from Win dows SEH The first pass runs before f i n a l ly blocks execute, mean ing that the locks held by the thread at the time of a throw are still held when up stack exception filters are run IL supports filters, although most C# developers are unaware because the language itself doesn't expose syntax for them (VB and VC++ do) Code inside of filters runs with locks held and can recursively acquire them
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