Re: [patch] Re: using long instead of atomic_t when only set/read is required

Alan Stern
2008-03-03T17:11:22Z

On Mon, 3 Mar 2008, Alan Cox wrote:

> > Atomicity of reads of write for pointers and integral types (other than
> > long long) should be documented.
>
> NAK.
>
> Atomicity of reads or writes for pointers and integral types is NOT
> guaranteed. Gcc doesn't believe in your guarantee.

Miscommunication and lack of clarity. CPU reads and writes _are_
guaranteed to be atomic. What is not guaranteed is that the compiler
will generate a single read or write instruction corresponding to a
particular expression in C.

Consider a routine like the following:

static task_struct *the_task;

void store_task(void)
{
the_task = current;
}

Is it possible to say whether readers examining "the_task" are
guaranteed to see a coherent value?

Alan Stern