On Wed, Mar 30, 2011 at 02:49:58PM -0700, Mingming Cao wrote:

quoted

Direct IO semantics have always been that the application is allowed
to overlap IO to the same range if it wants to. The result is
undefined (just like issuing overlapping reads and writes to a disk
at the same time) so it's the application's responsibility to avoid
overlapping IO if it is a problem.

I was thinking along the line to provide finer granularity lock to allow
concurrent direct IO to different offset/range, but to same offset, they
have to be serialized. If it's undefined behavior, i.e. overlapping is
allowed, then concurrent dio implementation is much easier. But not sure
if any apps currently using DIO aware of the ordering has to be done at
the application level. 

Yes, they're aware of it.  And they consider it a bug if they ever do
concurrent I/O to the same sector.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."