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FI G U R E 1 1 1 : Five d i n i n g Philosophers, each with his own chair, plate, a n d fork
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Unfortunately, the food being eaten is difficult (spaghetti), and requires two forks to be eaten Thankfully each philosopher can easily access two forks one to his left and one to his right-but this requires that two adjacent philosophers cannot be eating simultaneously If you haven't noticed the deadlock yet, here it is Imagine that, as a pro tocol, all philosophers begin eating by grabbing the left fork and then the right If a neighboring philosopher holds one of the forks, then the philoso pher in question must wait for his neighbor to put the fork down Now, imagine all philosophers decide to grab the left fork at once Each will suc ceed But now no forks are available! When each tries to grab the right fork, each will find it to be held by his neighbor and, hence, each philosopher must wait (indefinitely)
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Deadlocks without Locks Deadlocks have to d o with any kind o f "shared resource" and are not limited to locks There are even subtler ways in which a real deadlock might occur A single threaded apartment (STA), of the kind we discuss further in 1 6, Graphical User Interfaces, is equivalent to an exclusive lock Only one thread can update a GUI window or run code inside an apartment threaded COM at once And this STA lock can only be released by running messages in the queue, either by finishing the actively running callback or pumping the queue Failure to pump often leads to liveness problems, but not deadlock, such as a delay in processing messages But if some code
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running on the STA thread depends on code that is waiting to run on the STA thread (perhaps because it's been enqueued into the message queue) then a true deadlock could result The CLR pumps messages automatically during a wait, reducing the likelihood of this but it can show up in native code Even more obscure examples exist Here's a classic example of an STA induced deadlock A thread running in an STA generates a large quantity of apartment threaded COM component instances and their corresponding runtime callable wrappers (RCWs) These RCWs must be finalized by the CLR when they become unreachable, or they will leak But the CLR's final izer thread always joins the process's multithreaded apartment (MTA), meaning it must use a proxy that transitions to the STA in order to release the RCWs (according to COM's strict apartment rules) If the STA doesn't pump and dispatch the finalizer 's attempt to finalize the RCW, however perhaps because it has chosen to block using a nonpumping wait-the finalizer thread will be stuck It is blocked until the STA unblocks and pumps If the STA never pumps, the finalizer thread will never make any progress, and a slow, silent buildup of all finalizable resources will occur over time (see Further Reading, Brumme) This can, in turn, lead to a sub sequent out-of-memory crash or a process recycle in ASPNET Different types of deadlocks require different techniques to combat Most of this section focuses on lock based deadlocks exclusively because they are most common It is worth mentioning that CLR 20 introduced a managed debugging assistant (MDA), Cont extSwi t c h De a d l o c k, which monitors for deadlocks induced by cross-apartment proxies and failure to pump If a cross-apartment call takes longer than 60 seconds to complete, the CLR assumes the receiving STA is not pumping and fires this MDA
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Generally speaking, there are four conditions necessary for deadlock 1 Mutual exclusion Using a resource prevents all other threads from accessing it 2 Waiting After acquiring some resource, a thread may wait for another resource, which itself could be, at that moment, held exclu sively by another thread
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