[Mpi3-rma] Memory barriers in MPI_WIN_LOCK_ALL mode

[Pavan Balaji]

It's undefined without additional synchronization.  I'm doing additional 
synchronization in the example below.

  -- Pavan

On 10/29/2012 10:59 PM, Underwood, Keith D wrote:
> Doesn't that specific usage have undefined results?  (put/get targeting the same location during one epoch?)
>
>> -----Original Message-----
>> From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-
>> bounces at lists.mpi-forum.org] On Behalf Of Pavan Balaji
>> Sent: Monday, October 29, 2012 11:39 PM
>> To: mpi3-rma at lists.mpi-forum.org
>> Subject: [Mpi3-rma] Memory barriers in MPI_WIN_LOCK_ALL mode
>>
>>
>> Consider the following program:
>>
>> P0:
>> 	MPI_WIN_LOCK_ALL();
>> 	MPI_PUT(1, X, P1); /* Write 1 to variable X on P1 */
>> 	/* wave hand to P1 */
>>
>> P1:
>> 	MPI_WIN_LOCK_ALL();
>> 	/* wait for P0 to wave hand */
>> 	MPI_GET(X, P1);  /* local operation */
>>
>> If the MPI_GET on P1 does a local load operation internally, it is not memory
>> consistent without a memory barrier.  Does this mean that I need to always
>> do a memory barrier on all local GET operations to get the right value?
>>
>> Note that this inefficiency does not go away by replacing the waving of the
>> hand with an MPI_BARRIER for example, as it does not know which window,
>> if any, the synchronization is for.
>>
>> Note that this is likely only a theoretical exercise, since most (all?) compilers
>> will do a memory barrier anyway if they see a function call (MPI_GET in this
>> case).  But is MPI assuming that that's going to be the case for efficient
>> execution?
>>
>>    -- Pavan
>>
>> --
>> Pavan Balaji
>> http://www.mcs.anl.gov/~balaji
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>
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-- 
Pavan Balaji
http://www.mcs.anl.gov/~balaji