Thread synchronization with in the same address space

[Björn Döbel]

Am 10.10.2014 um 18:00 schrieb teclis High Elf:
> I have two threads running in the same server. On thread handles
> incoming ipc messages from the clients in different address spaces and
> the other thread does the work. The thread handling the incoming ipc
> needs to pass an integer value to the worker thread. Could I synchronize
> them using
>
> int l4util_cmpxchg32
> <https://os.inf.tu-dresden.de/L4Re/doc/group__l4util__atomic.html#gacb4a5e47fea67cc4c61b488fc374df62>
> (volatile l4_uint32_t
> <https://os.inf.tu-dresden.de/L4Re/doc/group__l4__basic__types.html#gac1d09f3e382e711b776931f10e6e1e5a>
> *dest, l4_uint32_t
> <https://os.inf.tu-dresden.de/L4Re/doc/group__l4__basic__types.html#gac1d09f3e382e711b776931f10e6e1e5a>
> cmp_val, l4_uint32_t
> <https://os.inf.tu-dresden.de/L4Re/doc/group__l4__basic__types.html#gac1d09f3e382e711b776931f10e6e1e5a>
> new_val)
>
> the worker thread could then check the value of dest to get the integer.
> Would the worker thread need to use an atomic read function when it
> reads the value of dest and if so which function would the worker thread
> use for the atomic read?

In your scenario, I would think of it like this:

int data = -1; // global variable, set to invalid value

server_thread()
{
    int val = receive_message();
    data = some_processing(val);
}

worker_thread()
{
     while (data == -1) // spin until data is valid
       ;
     /* Use data here */
}

I don't see the need for an atomic cmpxchg() in your specific case. 
Actually I would however suggest you use a pthread_mutex or a 
pthread_cond_var to synchronize your threads (or even IPC between the 
two...).

Bjoern