Re: ufsstat - testers / feedback wanted!

On Fri, 2005-Oct-14 10:10:04 +0100, Brian Candler wrote:
>I'd be grateful if you could clarify that point for me. Are you saying that
>if I write
>
>    long long foo;
>    ...
>    foo++;
>
>then the C compiler generates code for 'foo++' which is not thread-safe?
>(And therefore I would have to protect it with a mutex or critical section)

foo++ is not normally thread-safe even if foo is an int or pointer.

Typical iA32 code looks like:
	addl	$1,foo
This is only thread-safe in a UP environment.

Typical RISC code looks like:
	load	foo,%reg
	add	$1,%reg
	store	%reg,foo
This is not thread-safe.

Note that the compiler may keep foo in a register for an extended period
unless you convince the compiler not to.  Especially on RISC processors,
the compiler will normally spread the load/add/store to try and avoid
pipeline stalls.

If you share foo between two threads, you need to use atomic operations
(see <machine/atomic.h>), mutexes or critical sections on all updates.
Note that you can do atomic 64-bit operations on iA32 (except 80386 and
80486) using a locked cmpxchg8b in a loop.

-- 
Peter Jeremy