[Lazarus] thread safe

Michael Schnell mschnell at lumino.de
Mon Jun 27 15:49:18 CEST 2011

On 06/27/2011 03:31 PM, Andrew Brunner wrote:
> Simply protecting a loop with a futex is not sufficient.  Assigning
> variables in a multi-threaded environment is not sufficient.
While I don't think we should this discuss in this context, I suggest 
that in the docs for the user of the object library (that was the 
starting point of the discussion), any type of thread-safety should 
include threads running on multiple processors.
> Interlocked/Exchange/ExchangeAdd/Increment features must be used
> whenever dealing with variables (with at least one exception)  In my
> experience arrays of boolean are atomic.
Yep. But this is only relevant if you don't protect them by a critical 
section, as this excludes concurrent exchange before the critical 
instructions are executed. Of course (when possible) using the 
interlocked instruction results in a lot less overhead than simply using 
a critical section.
> I ran tests on a HexCore AMD system which had unexplained anomalies
> with pointers in an bidirectional linked list.  I replaced all the
> pointer assignments with InterlockedExchange and the system worked
> flawlessly.
This is clear. You also would have been able to avoid the problem by 
using critical sections around *all *the appropriate instructions, but 
supposedly this would have slowed done the application greatly.
>   Some of the assignments were taking place in a
> criticalsection (btw).
I believe that the "some" were not enough.

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