In the posted irecv case if you are reading from the posted receive buffer the problem is you may get one of three values:

1.  pre irecv value
2.  value received from the irecv in progress
3.  possibly garbage if you are unlucky enough to access memory that is at the same time being updated. 

--td
Alberto Canestrelli wrote:
Thanks,
it was late in the night yesterday and i highlighted STORES but I meanted to highlight LOADS! I know that
stores are not allowed when you are doing non blocking send-recv. But I was impressed about LOADS case. I always do some loads of the data
between all my ISEND-IRECVs and my WAITs. Could  you please confirm me that OMPI can handle the LOAD case? And if it cannot handle it, which could be the consequence? What could happen in the worst of the case when there is a data race in reading a data?
thanks
alberto

Il 02/08/2010 9.32, Alberto Canestrelli ha scritto:
I believe it is definitely a no-no to STORE (write) into a send buffer
while a send is posted. I know there have been debate in the forum to
relax LOADS (reads) from a send buffer. I think OMPI can handle the
latter case (LOADS). On the posted receive side you open yourself up
for some race conditions and overwrites if you do STORES or LOADS from a
posted receive buffer.

--td

Alberto Canestrelli wrote:
 Hi,
 I have a problem with a fortran code that I have parallelized with
 MPI. I state in advance that I read the whole ebook "Mit Press - Mpi -
 The Complete Reference, Volume 1" and I took different MPI classes, so
 I have a discrete MPI knowledge. I was able to solve by myself all the
 errors I bumped into but now I am not able to find the bug of my code
 that provides erroneous results. Without entering in the details of my
 code, I think that the cause of the problem could be reletad to the
 following aspect highlighted in the above ebook (in the follow I copy
 and paste from the e-book):

 A nonblocking post-send call indicates that the system may start
 copying data
 out of the send buffer. The sender must not access any part of the
 send buffer
 (neither for loads nor for STORES) after a nonblocking send operation
 is posted until
 the complete send returns.
 A nonblocking post-receive indicates that the system may start writing
 data into
 the receive buffer. The receiver must not access any part of the
 receive buffer after
 a nonblocking receive operation is posted, until the complete-receive
 returns.
 Rationale. We prohibit read accesses to a send buffer while it is
 being used, even
 though the send operation is not supposed to alter the content of this
 buffer. This
 may seem more stringent than necessary, but the additional restriction
 causes little
 loss of functionality and allows better performance on some systems-
 consider
 the case where data transfer is done by a DMA engine that is not
 cache-coherent
 with the main processor.End of rationale.

 I use plenty of nonblocking post-send in my code. Is it really true
 that the sender must not access any part of the send buffer not even
 for STORES? Or was it a MPI 1.0 issue?
 Thanks.
 alberto
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