Re: MMIO and gcc re-ordering issue

Trent Piepho
2008-06-04T06:40:21Z

On Wed, 4 Jun 2008, Nick Piggin wrote:
> On Wednesday 04 June 2008 07:44, Trent Piepho wrote:
>> On Tue, 3 Jun 2008, Matthew Wilcox wrote:
>
>>> I don't understand why you keep talking about DMA. Are you talking
>>> about ordering between readX() and DMA? PCI proides those guarantees.
>>
>> I guess you haven't been reading the whole thread. The reason it started
>> was because gcc can re-order powerpc (and everyone else's too) IO accesses
>> vs accesses to cachable memory (but not spin-locks), which ends up only
>> being a problem with coherent DMA.
>
> I don't think it is only a problem with coherent DMA.
>
> CPU0 CPU1
> mutex_lock(mutex);
> writel(something, DATA_REG);
> writel(GO, CTRL_REG);
> started = 1;
(A)
> mutex_unlock(mutex);
> mutex_lock(mutex);
(B)
> if (started)
> /* oops, this can reach device before GO */
> writel(STOP, CTRL_REG);

The locks themselves should have (and do have) ordering operations to insure
gcc and/or the cpu can't move a store or load outside the locked region.
Generally you need that to keep stores/loads to cacheable memory inside the
critical area, much less I/O operations. Otherwise all you have to do is
replace writel(something, ...) with shared_data->something = ... and there's
an obvious problem. In your example, gcc currently can and will move the GO
operation to point A (if it can figure out that CTRL_REG and started aren't
aliased), but that's not a problem. If it could move it to B that would be a
problem, but it can't.

Other than coherent DMA, I don't think there is any reason to care if I/O
accessors are strongly ordered wrt load/stores to cacheable memory. locking
and streaming DMA sync operations already need to have ordering, so they don't
require all I/O to be ordered wrt all cacheable memory.