问题
I am currently wading through the ARM architecture manual for the ARMv7 core. In chapter A3.5.3 about atomicity of memory accesses, it states:
If a single-copy atomic load overlaps a single-copy atomic store and for any of the overlapping bytes the load returns the data written by the write inserted into the Coherence order of that byte by the single-copy atomic store then the load must return data from a point in the Coherence order no earlier than the writes inserted into the Coherence order by the single-copy atomic store of all of the overlapping bytes.
As non-native english speaker I admit that I am slightly challenged in understanding this sentence.
Is there a scenario where writes to a memory byte are not inserted in the Coherence Order and thus the above does not apply? If not, am I correct to say that shortening and rephrasing the sentence to the following:
If the load happens to return at least one byte of the the write, then the load must return all overlapping bytes from a point no earlier than where the write inserted them into the Coherence order of all of the overlapping bytes.
still transports the same meaning?
回答1:
I see that wording in the ARMv8 ARM, which really tries to remove any possible ambiguity in a lot of places (even if it does make the memory ordering section virtually unreadable).
In terms of general understanding (as opposed to to actually implementing the specification), a little bit of ambiguity doesn't always hurt, so whilst it fails to make it absolutely clear what a "memory location" means, I think the old v7 manual (DDI0406C.b) is a nicer read in this case:
A read or write operation is single-copy atomic if the following conditions are both true:
After any number of write operations to a memory location, the value of the memory location is the value written by one of the write operations. It is impossible for part of the value of the memory location to come from one write operation and another part of the value to come from a different write operation
When a read operation and a write operation are made to the same memory location, the value obtained by the read operation is one of:
- the value of the memory location before the write operation
- the value of the memory location after the write operation.
It is never the case that the value of the read operation is partly the value of the memory location before the write operation and partly the value of the memory location after the write operation.
So your understanding is right - the defining point of a single-copy atomic operation is that at any given time you can only ever see either all of it, or none of it.
There is a case in v7 whereby (if I'm interpreting it right) two normally single-copy atomic stores that occur to the same location at the same time but with different sizes break any guarantee of atomicity, so in theory you could observe some unexpected mix of bytes there - this looks to have been removed in v8.
来源：https://stackoverflow.com/questions/24010989/arm-single-copy-atomicity