[fpc-devel] Threading support and C library under Linux/Unix

[Michael Schnell]

On 06/24/2010 11:05 AM, Henry Vermaak wrote:
>
> /* Atomic compare and exchange.  These sequences are not actually atomic;
>    there is a race if *MEM != OLDVAL and we are preempted between the two
>    swaps.  However, they are very close to atomic, and are the best that a
>    pre-ARMv6 implementation can do without operating system support.
>    LinuxThreads has been using these sequences for many years.  */
>
>   

:) :) :) "Very close to atomic" is funny, regarding that the (obviously
rare) case of non-atomic behavior will cause a system crash.

> The linux specific code is here..
>   

I suppose this uses the said "atomic region" method:

 - Userspace (without doing a change to system mode) calls a function
provided by the system in a dedicated place (in user-space, write
protected and common for all processes)
 - if no interrupt happens everything is fine
 - the interrupt code in the Kernel detects if the interrupted user
space function was preempted in the middle of such a function (i.e. PC
in the atomic area)
 - if such a condition is detected the kernel correctly completes the
function for the user space process and adjusts the user space PC before
returning to user space into this or another procedure / thread.

With such systems the C library (i.e. macros in .h files) for
"atomic_..."-functions is provided as a call to the appropriate "atomic
region"  function.

Of course this can be done directly in the RTL, as the addresses of the
"atomic_..."-functions is supposed to be stable.

-Michael