>A volatile write operation prevents earlier memory operations on the thread from being reordered to occur after the volatile write. A volatile read operation prevents later memory operations on the thread from being reordered to occur before the volatile read
Looks like release/acquire to me? A total ordering would be sequential consistency.
"In C#, using the volatile modifier on a field guarantees that every access to that field is a volatile memory operation"
This makes it sound like you are right and the volatile keyword has the same behaviour as the Volatile class which explicitly says it has acquire-release ordering.
But that seems to contradict "The volatile keyword doesn't provide atomicity for operations other than assignment, doesn't prevent race conditions, and doesn't provide ordering guarantees for other memory operations." from the volatile keyword documentation?
I too interpretat those docs as contradictory, and I wonder if, like how Java 5 strengthened volatile semantics, this happened at some point in C# too and the docs weren't updated? Either way the specification, which the docs say is definitive, says it's acquire/release.
"When a field_declaration includes a volatile modifier, the fields introduced by that declaration are volatile fields. [...] For volatile fields, such reordering optimizations are restricted:
A read of a volatile field is called a volatile read. A volatile read has “acquire semantics”; that is, it is guaranteed to occur prior to any references to memory that occur after it in the instruction sequence.
A write of a volatile field is called a volatile write. A volatile write has “release semantics”; that is, it is guaranteed to happen after any memory references prior to the write instruction in the instruction sequence."
Acquire-release ordering provides ordering guarantees for all memory operations. If an acquire observes a releases, the thread is also guaranteed to see all the previous writes done by the other thread - regardless of the atomicity of those writes. (There still can't be any other data races though.)
This volatile keyword appears to only consider that specific memory location whereas the Volatile class seem to implement acquire-release.
Somewhat off topic, but what is a realistic example of where you need atomics with sequential consistency? Like, what useful data structure or pattern requires it? I feel like I've seen every other ordering except that one (and consume) in real world code.
A mutex would be the most trivial example. I don't believe that is possible to implement, in the general case, with only acquire-release.
Sequential consistency mostly become relevant when you have more than two threads interacting with both reads and writes. However, if you only have single-consumer (i.e. only one thread reading) or single-producer (i.e. only one thread writing) then the acquire-release semantics ends up becoming sequential since the single-consumer/producer implicitly enforces a sequential ordering. I can potentially see some multi-producer multi-consumer queues lock-free queues needing sequential atomics.
I think it's rare to see atomics with sequential consistency in practice since you typically either choose (1) a mutex to simplify the code at the expense of locking or (2) acquire-release (or weaker) to minimize the synchronization.
No, sorry. I was just remembering where I've typically seen sequential consistency being used. For instance, Peterson's algorithm was what I had in mind. Spinlock is indeed a good example (although a terrible algorithm which I hope you haven't seen used in practice) of a mutex algorithm which only requires acquire-release.
Looks like release/acquire to me? A total ordering would be sequential consistency.