Re: Race condition between aio_complete and aio_read_evt

[Stephen Hemminger <shemminger@osdl.org>]

On Tue, 8 Jul 2003 14:52:28 -0700
"Chen, Kenneth W" <kenneth.w.chen@intel.com> wrote:

> We hit a memory ordering race condition on AIO ring buffer tail pointer
> between function aio_complete() and aio_read_evt().
> 
> What happens is that on an architecture that has a relaxed memory
> ordering model like IPF(ia64), explicit memory barrier is required in a
> SMP execution environment.  Considering the following case:
> 
> 1 CPU is executing a tight loop of aio_read_evt.  It is pulling event
> off the ring buffer.  During that loop, another CPU is executing
> aio_complete() where it is putting event into the ring buffer and then
> update the tail pointer.  However, due to relaxed memory ordering model,
> the tail pointer can be visible before the actual event is being
> updated.  So the other CPU sees the updated tail pointer but picks up a
> staled event data.
> 
> A memory barrier is required in this case between the event data and
> tail pointer update.  Same is true for the head pointer but the window
> of the race condition is nil.  For function correctness, it is fixed
> here as well.
> 
> By the way, this bug is fixed in the major distributor's kernel on 2.4.x
> kernel series for a while, but somehow hasn't been propagated to 2.5
> kernel yet.
> 
> The patch is relative to 2.5.74.
> 
> - Ken
> 

Make those smp_* memory barrier's because they don't matter on UP.