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From: Gleb Natapov (glebn_at_[hidden])
Date: 2007-06-13 09:27:19

Hello everyone,

  I encountered a problem with openib on depend connection code. Basically
it works only by pure luck if you have more then one endpoint for the same
proc and sometimes breaks in mysterious ways.

The algo works like this: A wants to connect to B so it creates QP and sends it
to B. B receives the QP from A and looks for endpoint that is not yet associated
with remote endpoint, creates QP for it and sends info back. Now A receives
the QP and goes through the same logic as B i.e looks for endpoint that is not
yet connected, BUT there is no guaranty that it will find the endpoint that
initiated the connection in the first place! And if it finds another one it will
create QP for it and will send it back to B and so on and so forth. In the end
I sometimes receive a peculiar mesh of connection where no QP has a connection
back to it from the peer process.

To overcome this problem B needs to send back some info that will allow A to
determine the endpoint that initiated a connection request. The lid:qp pair
will allow for this. But even then the problem will remain if two procs initiate
connection at the same time. To dial with simultaneous connection asymmetry
protocol have to be used one peer became master another slave. Slave alway
initiate a connection to master. Master choose local endpoint to satisfy
incoming request and sends info back to a slave. If master wants to initiate a
connection it send message to a slave and slave initiate connection back to

Included patch implements an algorithm described above and work for all
scenarios for which current code fails to create a connection.