Is accessing a variable in C# an atomic operation?

For the definitive answer go to the spec. :)

Partition I, Section 12.6.6 of the CLI spec states: "A conforming CLI shall guarantee that read and write access to properly aligned memory locations no larger than the native word size is atomic when all the write accesses to a location are the same size."

So that confirms that s_Initialized will never be unstable, and that read and writes to primitve types smaller than 32 bits are atomic.

In particular, double and long (Int64 and UInt64) are not guaranteed to be atomic on a 32-bit platform. You can use the methods on the Interlocked class to protect these.

Additionally, while reads and writes are atomic, there is a race condition with addition, subtraction, and incrementing and decrementing primitive types, since they must be read, operated on, and rewritten. The interlocked class allows you to protect these using the CompareExchange and Increment methods.

Interlocking creates a memory barrier to prevent the processor from reordering reads and writes. The lock creates the only required barrier in this example.

This is a (bad) form of the double check locking pattern which is not thread safe in C#!

There is one big problem in this code:

s_Initialized is not volatile. That means that writes in the initialization code can move after s_Initialized is set to true and other threads can see uninitialized code even if s_Initialized is true for them. This doesn't apply to Microsoft's implementation of the Framework because every write is a volatile write.

But also in Microsoft's implementation, reads of the uninitialized data can be reordered (i.e. prefetched by the cpu), so if s_Initialized is true, reading the data that should be initialized can result in reading old, uninitialized data because of cache-hits (ie. the reads are reordered).

For example:

Thread 1 reads s_Provider (which is null)  
Thread 2 initializes the data  
Thread 2 sets s\_Initialized to true  
Thread 1 reads s\_Initialized (which is true now)  
Thread 1 uses the previously read Provider and gets a NullReferenceException

Moving the read of s_Provider before the read of s_Initialized is perfectly legal because there is no volatile read anywhere.

If s_Initialized would be volatile, the read of s_Provider would not be allowed to move before the read of s_Initialized and also the initialization of the Provider is not allowed to move after s_Initialized is set to true and everything is ok now.

Joe Duffy also wrote an Article about this problem: Broken variants on double-checked locking

Hang about -- the question that is in the title is definitely not the real question that Rory is asking.

The titular question has the simple answer of "No" -- but this is no help at all, when you see the real question -- which i don't think anyone has given a simple answer to.

The real question Rory asks is presented much later and is more pertinent to the example he gives.

Why is the s_Initialized field read outside of the lock?

The answer to this is also simple, though completely unrelated to the atomicity of variable access.

The s_Initialized field is read outside of the lock because locks are expensive.

Since the s_Initialized field is essentially "write once" it will never return a false positive.

It's economical to read it outside the lock.

This is a low cost activity with a high chance of having a benefit.

That's why it's read outside of the lock -- to avoid paying the cost of using a lock unless it's indicated.

If locks were cheap the code would be simpler, and omit that first check.

(edit: nice response from rory follows. Yeh, boolean reads are very much atomic. If someone built a processor with non-atomic boolean reads, they'd be featured on the DailyWTF.)

The correct answer seems to be, "Yes, mostly."

1. John's answer referencing the CLI spec indicates that accesses to variables not larger than 32 bits on a 32-bit processor are atomic.
2. Further confirmation from the C# spec, section 5.5, Atomicity of variable references:

Reads and writes of the following data types are atomic: bool, char, byte, sbyte, short, ushort, uint, int, float, and reference types. In addition, reads and writes of enum types with an underlying type in the previous list are also atomic. Reads and writes of other types, including long, ulong, double, and decimal, as well as user-defined types, are not guaranteed to be atomic.

3. The code in my example was paraphrased from the Membership class, as written by the ASP.NET team themselves, so it was always safe to assume that the way it accesses the s_Initialized field is correct. Now we know why.

Edit: As Thomas Danecker points out, even though the access of the field is atomic, s_Initialized should really be marked volatile to make sure that the locking isn't broken by the processor reordering the reads and writes.

The Initialize function is faulty. It should look more like this:

private static void Initialize()
{
    if(s_initialized)
        return;

    lock(s_lock)
    {
        if(s_Initialized)
            return;
        s_Initialized = true;
    }
}

Without the second check inside the lock it's possible the initialisation code will be executed twice. So the first check is for performance to save you taking a lock unnecessarily, and the second check is for the case where a thread is executing the initialisation code but hasn't yet set the s_Initialized flag and so a second thread would pass the first check and be waiting at the lock.

I think you're asking if s_Initialized could be in an unstable state when read outside the lock. The short answer is no. A simple assignment/read will boil down to a single assembly instruction which is atomic on every processor I can think of.

I'm not sure what the case is for assignment to 64 bit variables, it depends on the processor, I would assume that it is not atomic but it probably is on modern 32 bit processors and certainly on all 64 bit processors. Assignment of complex value types will not be atomic.

Reads and writes of variables are not atomic. You need to use Synchronisation APIs to emulate atomic reads/writes.

For an awesome reference on this and many more issues to do with concurrency, make sure you grab a copy of Joe Duffy's latest spectacle. It's a ripper!

"Is accessing a variable in C# an atomic operation?"

Nope. And it's not a C# thing, nor is it even a .net thing, it's a processor thing.

OJ is spot on that Joe Duffy is the guy to go to for this kind of info. ANd "interlocked" is a great search term to use if you're wanting to know more.

"Torn reads" can occur on any value whose fields add up to more than the size of a pointer.

You could also decorate s_Initialized with the volatile keyword and forego the use of lock entirely.

That is not correct. You will still encounter the problem of a second thread passing the check before the first thread has had a chance to to set the flag which will result in multiple executions of the initialisation code.

I thought they were - I'm not sure of the point of the lock in your example unless you're also doing something to s_Provider at the same time - then the lock would ensure that these calls happened together.

Does that //Perform initialization comment cover creating s_Provider? For instance

private static void Initialize()
{
    if (s_Initialized)
        return;

    lock(s_lock)
    {
        s_Provider = new MembershipProvider ( ... )
        s_Initialized = true;
    }
}

Otherwise that static property-get's just going to return null anyway.

What you're asking is whether accessing a field in a method multiple times atomic -- to which the answer is no.

In the example above, the initialise routine is faulty as it may result in multiple initialization. You would need to check the s_Initialized flag inside the lock as well as outside, to prevent a race condition in which multiple threads read the s_Initialized flag before any of them actually does the initialisation code. E.g.,

private static void Initialize()
{
    if (s_Initialized)
        return;

    lock(s_lock)
    {
        if (s_Initialized)
            return;
        s_Provider = new MembershipProvider ( ... )
        s_Initialized = true;
    }
}

Perhaps Interlocked gives a clue. And otherwise this one i pretty good.

I would have guessed that their not atomic.

Ack, nevermind... as pointed out, this is indeed incorrect. It doesn't prevent a second thread from entering the "initialize" code section. Bah.

You could also decorate s_Initialized with the volatile keyword and forego the use of lock entirely.