On 1/11/21 8:49 PM, tsbockman wrote:
On Monday, 11 January 2021 at 00:43:00 UTC, Tim wrote:
When MessageService calls the delegate for start, db is null. If I
call start() in the Foo constructor it works just fine. Am I missing
something here? Do delegates get called outside of their class
context? I know I could just pass the db into start but I want to work
out exactly why this is happening
The compiler and the physical CPU are both allowed to change the order
in which instructions are executed to something different from what your
code specifies, as long as the visible, "official" results and effects
of the chosen order of execution are the same as those of your specified
code, FROM THE PERSPECTIVE OF THE EXECUTING THREAD.
This is allowed so that the compiler can optimize to minimize negative
"unofficial" effects such as the passage of time and memory consumption.
However, this re-ordering IS permitted to freely alter the behavior of
your code from the perspective of OTHER threads. A likely cause of your
bug is that the write to db by the constructor's thread is being
committed to memory after the read of db by the MessageService thread.
I don't think this is valid.
1. the compiler MUST NOT reorder the storage of db to after you pass a
delegate into an opaque function (array allocation).
2. The CPU is not going to reorder, because the memory allocation is
going to take a global lock anyway (mutex locks should ensure memory
consistency).
I think something weird is going on, but I don't know what.
In order to RELIABLY fix this kind of problem, you must correctly use
the only commands which the compiler and CPU are NOT allowed to reorder
with respect to other threads, namely atomic operations, memory barriers
and synchronization primitives.
I can't ever imagine creating a thread (which is likely what
MessageService ctor is doing) to not have a consistent memory with the
creating thread on construction. Why would the other thread not see the
same memory when it didn't exist before? The CPU would have to go out of
its way to make it inconsistent.
-Steve
