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    <p>You are locking on the instances themselves and requiring that both instances be locked to transfer something. That is a recipe for deadlocks.</p> <ul> <li>Thread 1 locks on acc1 to begin a transfer</li> <li>Thread 2 locks on acc2 to begin a transfer</li> <li>Thread 1 waits for the lock on acc2 to be released so it can complete its transfer</li> <li>Thread 2 waits for the lock on acc1 to be released so it can complete its transfer</li> </ul> <p>They will each wait for each other to let go of their lock indefinitely.</p> <p>If you <em>have</em> to acquire multiple locks at once, <em>always</em> acquire the locks in the same order. That said, try your best not to need more than one lock at once by changing your object responsibilities. </p> <p>For example, withdrawal and depositing are two separate actions that aren't related, but they modify the balance. You are trying to protect <em>the balance</em> with locks. Once an account's balance is changed, there's no point in keeping that lock anymore. Additionally, I would suggest that it's not the responsibility of an account to know how to transfer to other accounts. </p> <p>With that in mind, here are changes that eliminate the deadlock.</p> <pre><code>type Account(name:string) = let mutable balance = 1000 let accountSync = new Object() member x.Withdraw amount = lock accountSync (fun () -&gt; balance &lt;- balance - amount) member x.Deposit amount = lock accountSync (fun () -&gt; balance &lt;- balance + amount) let transfer amount (fromAccount:Account) (toAccount:Account) = fromAccount.Withdraw(amount) toAccount.Deposit(amount) </code></pre>
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    1. USChris Hannon
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    1. COI'm confused why you use a new Object to do locking while we can just lock the class object itself (i.e. lock x). I know your code works but I'm just not entirely sure why.
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      1. USuser2431438
    2. CO@user2431438 It's a preventative measure. As far as functionality goes, it really doesn't matter what instance you lock on as long as it's _the same instance_ for all operations affecting a given thing (in this case, the balance). Locking on the class instance itself, though, allows _anyone who can see the instance_ to lock it, potentially affecting the internal ability of the Account type to manage itself effectively (for example, somebody deadlocks on that instance and now that account is frozen). Creating a private instance within the type ensures that only the Account type can use it.
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      1. USChris Hannon
    3. CO@user2431438: You should not lock externally visible objects because users of your library also can. Therefore, it is good practice to use a private object as Chris has.
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      1. USJon Harrop
 

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