08.12.2015 16:29, Jim Starkey wrote:
> In virtual all cases involving updates of non-interlocked data structures,
> the logic is --
> and must be:
Unfortunately, this logic doesn't work quite well for bit flags.
Consider two threads, setting flags A and B respectively.
> for (;;)
> {
> <examine state making tentative decision and computer desired future
> state>
Here thread 1 got value with both flags unset.
> if (<state incompatible with operation>)
> <punt>
In meantime thread 2 set flag B.
> if (<interlocked compare and swap>)
Here thread 1 set the value with flag A set and flag B reset.
> <success -- break out of loop or whatever>
Check fail, but still flag B is not set until the next round. The window for
races is
small but still exists.
For integer flags situation is better. If we want to set flag, we just do
if (interlocked compare and swap)
<failure -- someone already set the flag before us, abort the action>
Still there is a hope that CPU manufacturers will someday implement
atomic_fetch_and_or
in hardware because it is engraved in C++11 standard.
--
WBR, SD.
------------------------------------------------------------------------------
Go from Idea to Many App Stores Faster with Intel(R) XDK
Give your users amazing mobile app experiences with Intel(R) XDK.
Use one codebase in this all-in-one HTML5 development environment.
Design, debug & build mobile apps & 2D/3D high-impact games for multiple OSs.
http://pubads.g.doubleclick.net/gampad/clk?id=254741911&iu=/4140
Firebird-Devel mailing list, web interface at
https://lists.sourceforge.net/lists/listinfo/firebird-devel
