David Howells <[email protected]> writes:
[...]
> +=======================================
> +LINUX KERNEL COMPILER BARRIER FUNCTIONS
> +=======================================
> +
> +The Linux kernel has an explicit compiler barrier function that prevents the
> +compiler from moving the memory accesses either side of it to the other side:
> +
> + barrier();
> +
> +This has no direct effect on the CPU, which may then reorder things however it
> +wishes.
> +
> +In addition, accesses to "volatile" memory locations and volatile asm
> +statements act as implicit compiler barriers.
This last statement seems to contradict with what GCC manual says about
volatile asm statements:
"You can prevent an `asm' instruction from being deleted by writing the
keyword `volatile' after the `asm'. [...]
The `volatile' keyword indicates that the instruction has important
side-effects. GCC will not delete a volatile `asm' if it is reachable.
(The instruction can still be deleted if GCC can prove that
control-flow will never reach the location of the instruction.) *Note
that even a volatile `asm' instruction can be moved relative to other
code, including across jump instructions.*"
I think that volatile memory locations aren't compiler barriers either,
-- GCC only guarantees that it won't remove the access and that it won't
re-arrange the access w.r.t. other *volatile* accesses. On the other
hand, barrier() indeed prevents *any* memory access from being moved
across the barrier.
-- Sergei.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
