[fpc-devel] TMultiReadExclusiveWriteSynchronizer does not allow "read lock" promoting to a "write lock"

[Nikolai Zhubr]

19.02.2010 23:02, Inoussa OUEDRAOGO:
> Hi
>
> The TMultiReadExclusiveWriteSynchronizer implementation does not allow
> "read lock" promoting
> to a "write lock". The program above hang in FPC 2.4 and 2.5.x while
> working in Delphi and
> FPC 2.2.x. It hangs at the "x.Beginwrite();" instruction. The
> implementation clearly does
> not support this scenario. It will be helpfull to note this in the
> documentation.
AFAIK entering exclusive lock without releasing multiple-read lock is 
very unsafe regarding deadlocks (Almost deadlock-guaranteed). Of course 
it is safe in the example, but why bother about special 1-thread 
scenario if multithreaded code will be unsafe anyway?

Nikolai

>
> <code>
> program test;
>
> {$mode objfpc}{$H+}
>
> uses
>    SysUtils;
>
> var
>    x : TMultiReadExclusiveWriteSynchronizer;
> begin
>    x := TMultiReadExclusiveWriteSynchronizer.Create();
>    x.Beginread();
>      WriteLn('Beginread');
>      x.Beginwrite();
>        WriteLn('Beginwrite');
>      x.Endwrite();
>        WriteLn('Endwrite');
>    x.Endread();
>      WriteLn('Endread');
>
>    ReadLn;
> end.
> </code>
>