Volatile Violates its main job?

Question

According to MSDN:

The volatile keyword indicates that a field might be modified by multiple threads that are executing at the same time. Fields tha

Answer 1

The MSDN documentation is wrong. That is most certainly not what volatile does. The C# specification tells you exactly what volatile does and getting a "fresh read" or a "committed write" is not one of them. The specification is correct. volatile only guarantees acquire-fences on reads and release-fences on writes. These are defined as below.

• acquire-fence: A memory barrier in which other reads and writes are not allowed to move before the fence.
• release-fence: A memory barrier in which other reads and writes are not allowed to move after the fence.

I will try to explain the table using my arrow notation. A ↓ arrow will mark a volatile read and a ↑ arrow will mark a volatile write. No instruction can move through the arrowhead. Think of the arrowhead as pushing everything away.

In the following analysis I will use to variables; x and y. I will also assume that they are marked as volatile.

Case #1

Notice how the placement of the arrow after the read of x prevents the read of y from moving up. Also notice that the volatility of y is irrelevant in this case.

var localx = x;
↓
var localy = y;
↓

Case #2

Notice how the placement of the arrow after the read of x prevents the write to y from moving up. Also notice that the volatility of either of x or y, but not both, could have been omitted in this case.

var localx = x;
↓
↑
y = 1;

Case #3

Notice how the placement of the arrow before the write to y prevents the write to x from moving down. Notice that the volatility of x is irrelevant in this case.

↑
x = 1;
↑
y = 2;

Case #4

Notice that there is no barrier between the write to x and the read of y. Because of this the either the write to x can float down or the read of y can float up. Either movement is valid. This is why the instructions in the write-read case can be swapped.

↑
x = 1;
var localy = y;
↓

Notable Mentions

It is also important to note that:

• x86 hardware has volatile semantics on writes.
• Microsoft's implementation of the CLI (and suspect Mono's as well) has volatile semantics on writes.
• The ECMA specification does not have volatile semantics on writes.