I am reading about memory barriers and what I can summarize is that they prevent instruction re-ordering done by compilers.
So in User space memory lets say I have
b = 0;
main(){
a = 10;
b = 20;
c = add(a,b);
}
Can the compiler reorder this code so that b = 20 assignment happens after c = add() is called.
Why we do not use barriers in this case ? Am I missing some fundamental here.
Does Virtual memory is exempted from any re ordering ?
Extending the Question further:
In Network driver:
1742 /*
1743 * Writing to TxStatus triggers a DMA transfer of the data
1744 * copied to tp->tx_buf[entry] above. Use a memory barrier
1745 * to make sure that the device sees the updated data.
1746 */
1747 wmb();
1748 RTL_W32_F (TxStatus0 + (entry * sizeof (u32)),
1749 tp->tx_flag | max(len, (unsigned int)ETH_ZLEN));
1750
When he says devices see the updated data... How to relate this with the multi threaded theory for usage of barriers.