Re: Creating a thread-local duplicate of a globally shared array

[Andrej Mitrovic]

Thanks for the help. But it appears I've ran into some kind of other bug. ddbg:

Unhandled Exception: EXCEPTION_ACCESS_VIOLATION(0xc005) at __aaInX
(0x0041b616) thread(552)

That seems like the hash method for checking keys. I have a static
int[int] hash which I'm not sharing with other threads, I just check
if a key is in there. Damn, this will take some work to make a good
test case..

Re: Creating a thread-local duplicate of a globally shared array

[Steven Schveighoffer]

On Fri, 01 Jul 2011 01:14:26 -0400, Andrej Mitrovic  
 wrote:



I have two functions running concurrently and they share data via a
globally shared array.

Generally one thread modifies an array and potentially changes its
length, the other thread reads from it. I have to avoid too many locks
and message passing wouldn't really work since I need fast access to
the array.

I can't use the array directly in the reading thread because the array
could possibly be reallocated by the writing thread (e.g. if it
changes the .length property), while at the same time the reading
thread could just have sent the .ptr value of that array to some API
function. I've had this problem occur and the app would crash due to a
reallocation while the API was reading the array.

Here's the gist of it:

__gshared int[] values;

void foo()
{
// modify, write to the array, and possibly change the length
}


It's not quite safe (even with locks) to alter __gshared array lengths  
from more than one thread.  This is due to the assumption that any array  
not marked shared is thread local.  This includes __gshared data, since  
__gshared is not part of the type.


What can happen is the array block information can be cached in the thread  
local LRU append cache, and another thread could extend the data, thereby  
changing the block information.  When the first thread then tries to use  
the block information, he gets a stale block info from the runtime, and  
can make incorrect assumptions.


Have you tried changing __gshared to shared?  shared should be supported.

-Steve