[redixhumayun]

Hey, so I'm curious why having memory barriers span across threads is the wrong mental model.

Assuming that the memory barrier is syncing across a single variable (in this case ready), why would it be correct to think of it as two separate barriers? If it were correct to think of it as two separate barriers on two separate threads, wouldn't there need to be some form of synchronization or linkage between the two barriers themselves so that memory barriers can be coupled together?

For instance, if I had release-acquire models on two variables, ready and not_ready, using separate barriers as representation might look something like this

```

  Thread 1                Memory                  Thread 2
  ---------               -------                 ---------
  |                          |                          |
  |   write(data, 100)       |                          |
  | -----------------------> |                          |
  |                          |                          |
  | ====Memory Barrier====== |                          |
  |   store(ready, true)     |                          |
  | -----------------------> |                          |
  | ====Memory Barrier====== |                          |
  |                          |                          |
  | ====Memory Barrier====== |                          |
  |   store(not_ready, true) |                          |
  | -----------------------> |                          |
  | ====Memory Barrier====== |                          |
  |                          |                          |
  |                          | ===Memory Barrier======= |
  |                          |   load(ready) == true    |                   
  |                          | <----------------------  |
  |                          | ====Memory Barrier=====  |
  |                          |                          |
  |                          |.===Memory Barrier======= |
  |                          |   load(not_ready) == true|                   
  |                          | <----------------------  |
  |                          | ====Memory Barrier=====  |
  |                          |                          |
  |                          |       read(data)         |
  |                          | <----------------------  |
  |                          |                          |

```

Now, how does the processor know which memory barriers are linked together? I ask because without understanding which barriers are linked together, how is instruction re-ordering determined?

[gpderetta]

The linking of barriers in pair is really just a mental model, not (usually) what happens at the hardware level. In fact in the C++ memory model the synchronizes-with relationship is load and stores, not barriers, which indirectly affect the properties of load and stores around them. That's another reason why I don't really like the memory barrier model and I prefer to think in terms of happens-before dependency graphs.

edit: AFAIK, seq_cst ordering (as opposed to acq_rel) is only relevant when you have more than two threads and you care about things like IRIW. In this case acquires and releases are not enough to capture the full set of constraints, although at the hardware level it is still everything local.

edit2: I guess the missing bit is that beyond the hardware fences you have the hardware cache coherency protocol that makes sure that a total order of operations always exist once load and stores reach the coherence fabric.

[redixhumayun]

Yeah, I see your point about thinking in terms of dependency graphs. I actually got the idea for using a visual memory barrier from the Linux docs(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...) and the C++ concurrency in action book.

>I guess the missing bit is that beyond the hardware fences you have the hardware cache coherency protocol that makes sure that a total order of operations always exist once load and stores reach the coherence fabric.

Can you explain more about this?

[bluetomcat]

Modern processors are out-of-order execution beasts. A barrier within a thread serves to enforce some ordering within that thread - that a store will occur after another store, and that a load will occur before another load. Threads know nothing of each other.