Archive for the ‘Deadlock’ Tag

The Parable of the Bathroom

Time and time again I find myself having to explain why people shouldn’t take a gung-ho approach to locking, or, more often, what a deadlock is. Simple explanations being the best, the following is the stock answer I came up with for these situations.

The first parable of the bathroom

Imagine a typical office, with two office workers. We’ll call these people Mr. A and Mr B.

Mr. A spills some coffee, and wants to wipe it up. He goes to the bathroom, grabs the toilet roll, and starts mopping up the spill.

While Mr. A is busy, Mr. B goes to answer the call of nature, and goes into the bathroom. Mr. B obviously likes his privacy, so he locks the bathroom door so that no one can enter while he’s using it.

Now, both A and B are stuck. When A finishes mopping up the coffee, he cannot replace the toilet roll because B has locked the door. On the other hand, B cannot finish before he uses the toilet roll, which he cannot do because the toilet roll simply isn’t in the room.

This is a typical deadlock situation – two workers are waiting for each other to finish, which will never happen because their end conditions are interdependent.

The second (unimproved) parable of the bathroom

Now, some forms of locking provided by .Net allow the developer to specify a timeout. This tells the worker to give up if it is unable to acquire a lock within the given time. In our example, this would looks something like this:

A goes into the bathroom. (A locks the bathroom)
A gets the toilet roll (A locks the toilet roll)
A leaves the bathroom (A releases the bathroom)
B enters the bathroom (B locks the bathroom)
B tries to get hold of the toilet roll (B tries to lock the toilet roll)

Of course, the lock on the toilet roll is still held by A, so B will give up trying to get the lock. Depending on how B is coded, it will either fail with an exception, or simply go ahead, unlock the bathroom, and continue working. Both situations are undesirable; in the first case, it means that B is not doing the work it was meant to do. In the second case, B continues working in a possibly inappropriate state (It left the bathroom without using the toilet roll).

Timeouts on locks are, in general, a BAD idea; so much in fact that the recommended locking syntax in C#, the lock statement, does not even give you an option to specify one.

The third (possibly improved, possibly not) parable of the bathroom

This still leaves us with a deadlock though. One way to get past it is to lock more aggressively, which may not be the ideal solution in most cases. It certainly works, but the chances are that it’s going to put the hurt on performance.

A more aggressive lock would be something like this:

A goes into the bathroom (A locks the bathroom)
A gets the toilet roll (A locks the toilet roll)
A leaves the bathroom, but keeps the key (A is still holding the lock to the bathroom)
B tries to get into the bathroom, and starts waiting for A to unlock it.
A returns the toilet roll. (A releases the toilet roll)
A returns the key to the bathroom. (A releases the bathroom)
B finishes waiting and goes about its business.

In this case, we are considering that to finish a given task (returning the toilet roll), A depends on the availability of the bathroom; to make sure this is available when needed, we lock the bathroom for the duration of the task, so a deadlock situation never arises – if A has the toilet roll, it also has exclusive access to the bathroom.

While this works, it is important to remember that locks should really be considered on a case by case basis. For example, while an aggressive lock as above may be acceptable if A has a brief task, it would not have been acceptable if the task had required a certain amount of time. Sometimes a reorganization of the code or the resources may be in order – again in the example above, it may not have been necessary to take out the entire toilet roll; perhaps taking a small part of it would have been enough.

Moral of the story

Handle locks with care, or you’ll end up in crap up to your eyeballs.

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