Thread 1 acquire lock for a on entering Thread 2 in Java

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Thread 1 acquire lock for a on entering Thread 2
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aswapValue(b) pass lock for a (since already held) on entering t = getValue() block waiting for lock for b on entering v = othergetValue()
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acquire lock for b on entering bswapValue(a) pass lock for b (since already held) on entering t = getValue() block waiting for lock for a on entering v = othergetValue()
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At this point both threads block forever
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More generally, deadlock is possible when two or more objects are mutually accessible from two or more threads, and each thread holds one lock while trying to obtain another lock already held by another thread
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226 Resource Ordering
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The need to preclude or recover from deadlocks and other liveness failures motivates the use of other exclusion techniques presented in this chapter However, one simple technique, resource ordering can be applied to classes such as Cell without otherwise altering their structure The idea behind resource ordering is to associate a numerical (or any other strictly orderable data type) tag with each object that can be held in a nested synchronized block or method If synchronization is always performed in least-first order with respect to object tags, then situations can never arise in which one thread has the synchronization lock for x while waiting for y and another has
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the lock for y while waiting for x Instead, they will both obtain the locks in the same order, thus avoiding this form of deadlock More generally, resource ordering can be used whenever there is a need to arbitrarily break symmetry or force precedence in a concurrent design In some contexts (see for example 245), there may be reasons to impose some specific ordering rules surrounding a set of locks But in others, you can use any convenient tag for lock-ordering purposes For example, you may be able to use the value returned by SystemidentityHashCode This method always returns the default implementation of ObjecthashCode, even if a class overrides the hashCode method While there is no guarantee that identityHashCode is unique, in practice run-time systems rely on codes to be distinct with a very high probability To be even safer about it, you could override method hashCode or introduce another tag method to ensure uniqueness in any classes employing resource ordering For example, you could assign each object a sequence number using one of the classes in 224 One further check, alias detection, can be applied in methods using nested synchronization to handle cases in which two (or more) of the references are actually bound to the same object For example, in swapValue, you can check whether a Cell is being asked to swap with itself This kind of check is strictly optional here (but see 251) Synchronization lock access is per-thread, not per-invocation Additional attempts to synchronize on already held objects will still work However, routine aliaschecking is a useful way to forestall downstream functionality, efficiency, and synchronization-based complications It may be applied before using synchronization surrounding two or more objects unless they are of distinct, unrelated types (Two references of two unrelated declared types cannot possibly be referring to the same object anyway, so there is no reason to check) A better version of swapValue, applying both resource ordering and alias detection, can be written as:
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public void swapValue(Cell other) { if (other == this) // alias check return; else if (SystemidentityHashCode(this) < SystemidentityHashCode(other)) thisdoSwapValue(other); else otherdoSwapValue(this); } protected synchronized void doSwapValue(Cell other) { // same as original public version: long t = getValue(); long v = othergetValue(); setValue(v); othersetValue(t); }
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As a minor efficiency tweak, we could further streamline the code inside doSwapValue first to acquire the necessary locks, and then directly access the value fields This avoids a self-call to a synchronized method while already holding the required lock, at the minor expense of adding lines of code that would need to be changed if the nature of the fields were ever modified:
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// slightly faster version protected synchronized void doSwapValue(Cell other) { synchronized(other) { long t = value; value = othervalue; othervalue = t; } }
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Note that the lock for this is obtained via the synchronized method qualifier, but the lock for other is explicitly acquired A further, very tiny (perhaps nonexistent) performance improvement might be obtained by folding the code in doSwapValue into swapValue, remembering to acquire both locks explicitly Lock-ordering problems are by no means restricted to methods using nested synchronization The issue arises in any code sequence in which a synchronized method holding the lock on one object in turn calls a synchronized method on another object However, there is less opportunity to apply resource ordering in cascaded calls: In the general case, one object cannot know for sure which other objects will be involved in downstream calls and whether they require synchronization This is one reason that deadlock can be such a hard problem in open systems (see 25) when you cannot release synchronization during calls (see 241)
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