问题
Like many other people, I've always been confused by volatile reads/writes and fences. So now I'm trying to fully understand what these do.
So, a volatile read is supposed to (1) exhibit acquire-semantics and (2) guarantee that the value read is fresh, i.e., it is not a cached value. Let's focus on (2).
Now, I've read that, if you want to perform a volatile read, you should introduce an acquire fence (or a full fence) after the read, like this:
int local = shared;
Thread.MemoryBarrier();
How exactly does this prevent the read operation from using a previously cached value? According to the definition of a fence (no read/stores are allowed to be moved above/below the fence), I would insert the fence before the read, preventing the read from crossing the fence and being moved backwards in time (aka, being cached).
How does preventing the read from being moved forwards in time (or subsequent instructions from being moved backwards in time) guarantee a volatile (fresh) read? How does it help?
Similarly, I believe that a volatile write should introduce a fence after the write operation, preventing the processor from moving the write forward in time (aka, delaying the write). I believe this would make the processor flush the write to the main memory.
But to my surprise, the C# implementation introduces the fence before the write!
[MethodImplAttribute(MethodImplOptions.NoInlining)] // disable optimizations
public static void VolatileWrite(ref int address, int value)
{
MemoryBarrier(); // Call MemoryBarrier to ensure the proper semantic in a portable way.
address = value;
}
Update
According to this example, apparently taken from "C# 4 in a Nutshell", fence 2 , placed after a write is supposed to force the write to be flushed to main memory immediately, and fence 3, placed before a read, is supposed to guarantee a fresh read:
class Foo{
int _answer;
bool complete;
void A(){
_answer = 123;
Thread.MemoryBarrier(); // Barrier 1
_complete = true;
Thread.MemoryBarrier(); // Barrier 2
}
void B(){
Thread.MemoryBarrier(); // Barrier 3;
if(_complete){
Thread.MemoryBarrier(); // Barrier 4;
Console.WriteLine(_answer);
}
}
}
The ideas in this book (and my own personal beliefs) seem to contradict the ideas behind C#'s VolatileRead and VolatileWrite implementations.
回答1:
How exactly does this prevent the read operation from using a previously cached value?
It does no such thing. A volatile read does not guarantee that the latest value will be returned. In plain English all it really means is that the next read will return a newer value and nothing more.
How does preventing the read from being moved forwards in time (or subsequent instructions from being moved backwards in time) guarantee a volatile (fresh) read? How does it help?
Be careful with the terminology here. Volatile is not synonymous with fresh. As I already mentioned above its real usefulness lies in how two or more volatile reads are chained together. The next read in a sequence of volatile reads will absolutely return a newer value than the previous read of the same address. Lock-free code should be written with this premise in mind. That is, the code should be structured to work on the principal of dealing with a newer value and not the latest value. This is why most lock-free code spins in a loop until it can verify that the operation completely successfully.
The ideas in this book (and my own personal beliefs) seem to contradict the ideas behind C#'s VolatileRead and VolatileWrite implementations.
Not really. Remember volatile != fresh. Yes, if you want a "fresh" read then you need to place an acquire-fence before the read. But, that is not the same as doing a volatile read. What I am saying is that if the implementation of VolatileRead had the call to Thread.MemoryBarrier before the read instruction then it would not actually produce a volatile read. If would produce fresh a read though.
回答2:
The important thing to understand is that volatile does not only mean "cannot cache value", but also gives important visibility guarantees (to be exact, it's entirely possible to have a volatile write that only goes to cache; depends solely on the hardware and its used cache coherency protocols)
A volatile read gives acquire semantics, while a volatile write has release semantics. An acquire fence means that you cannot reorder reads or writes before the fence, while a release fence means you cannot move them after the fence. The linked answer in the comments explains that actually quite nicely.
Now the question is, if we don't have any memory barrier before the load how is it guaranteed that we'll see the newest value? The answer to that is: Because we also put memory barriers after each volatile write to guarantee that.
Doug Lea wrote a great summary on which barriers exist, what they do and where to put them for volatile reads/writes for the JMM as a help for compiler writers, but the text is also quite useful for other people. Volatile reads and writes give the same guarantees in both Java and the CLR so that's generally applicable.
Source - scroll down to the "Memory Barriers" section (I'd copy the interesting parts, but the formatting doesn't survive it..)
来源：https://stackoverflow.com/questions/21652938/where-to-places-fences-memory-barriers-to-guarantee-a-fresh-read-committed-write