> I am running a server which essentially does the following operations in a
> loop:
>
> A[i].value = counter; //It's actually something else
> asm volatile ("" : : : "memory");
> asm volatile("mfence" ::: "memory");
> A[i].counter = counter;
> printf("%d %d\n", A[i].value, A[i].counter);
> counter ++;
>
> Basically, I want a fresh value of A[i].counter to indicate a fresh
> A[i].value.
>
> I have a remote client which reads the struct A[i] from the server
> (via RDMA) in a loop. Sometimes in the value that the client reads,
> A[i].counter is larger than A[i].value. i.e., I see the newer value of
> A[i].counter but A[i].value corresponds to a previous iteration of the
> server's loop.
>
> How can this happen in the presence of memory barriers? With barriers,
> A[i].counter should be updated later and therefore should always be
> smaller than A[i].value.
>
> Thanks for your help!
It seems possible for a remote read to start retrieving memory before an
update, such that A[i].value is read and placed on the wire, the server
modifies the memory, and then A[i].counter is read and placed on the wire. It
may depend on how large the data is that's being read and the RDMA read
implementation.
- Sean
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