Experimental Physics and
Industrial Control System

Tim Mooney <[email protected]> · Wed, 27 Oct 2010 11:54:05 -0500

One point that is probably obvious to you guys, but was obvious to me only in
retrospect,

is that it matters *where* in the processing chain the asynchronous record occurs.
If the only asynchronous record in the chain is the first one, you can get good
behavior

from a sequence of ca_put_callbacks and be fooled into thinking that server-side
code

is more aware of putNotify than it actually is. This is because the first
record's PACT field

will be True all the while the putNotify is active, so server-side code, which
only honors

record locks and PACT fields, will accidentally be honoring putNotify as well.
To make a good ca_put_callback test, you have to write to a synchronous record
that links

to an asynchronous record. In this case, when the first record has written to
the asynchronous

record, and that record has gone into its asynchronous wait, both the lockset
and the

first record's PACT field will look ready for another ca_put_callback, but the
putNotify

fields will not be ready, and you'll be able to see the problem you need to
defend against.
Tim Mooney
On 10/26/2010 7:04 PM, Jeff Hill wrote:

He said there are no combinations that will fail, but he's wrong about the
ca_put_callback() that you remember reading about, the operation can (and
does) fail in some circumstances.

One put callback failure mode is certainly that the server is blocked waiting for a put callback to finish for a very long time (60 seconds), the request eventually times out in the server, and the server returns an error to the client. Others are bad resource identifier, lack of write access, or a bad data type to be completely rigorous.
However that's not going to be true if there's any
asynchronous processing involved.  In that case the
putNotify can still be active when the process()
operation requested by RSRV thread returns, so the
server can try processing the second ca_put_callback()
operation but it may be rejected.

I will have to add a small correction to Andrews statement.
If the 2nd put callback is being started by the same client
(the usage case that Ben specifically called out) then the
per-client thread in the server will know that a put callback
is already in progress for that channel and it will certainly
wait for it to complete before starting a new put notify
request even if it is an asynchronous record.
If the 2nd put callback is being started by a different client
I think I recall (disclaimer: I am working form memory and I
didn't write the dbPutNotify code) correctly that the request
is placed on a list of outstanding requests by dbPutNotify and
these outstanding requests (each with their own private put
notify control block) are, each and every one of them, executed in
the order that they were placed in this queue - asynchronous
record processing or not.
So Andrew is correct that a put notify control block will not
allow a 2nd put notify for the _same_ put notify control block
to be started until the fist put notify (for the same control
block) completes, bit the ca server does not allow that situation
to happen.
Jeff
______________________________________________________
Jeffrey O. Hill           Email        [email protected]
LANL MS H820              Voice        505 665 1831
Los Alamos NM 87545 USA   FAX          505 665 5107
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-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Andrew Johnson
Sent: Tuesday, October 26, 2010 4:34 PM
To: [email protected]
Cc: Ben Franksen
Subject: Re: Some Channel Access Questions
Hi Ben,
I disagree with Jeff's response to this part of your question:
On Tuesday 26 October 2010 15:17:36 Ben Franksen wrote:

combinations which will fail ("sorry, pv is active")? I remember
reading some time ago that only one *db*_put_callback can be active at
a time on a record (on the IOC), so will interleaving two
ca_put_callbacks also fail?

He said there are no combinations that will fail, but he's wrong about the
ca_put_callback() that you remember reading about, the operation can (and
does) fail in some circumstances.
The IOC database can only have one putNotify operation active on a
particular
record at once.  The server side of a ca_put_callback() operation uses the
IOC's putNotify mechanism to process the record and discover when the
processing chain has completed, which is when it notifies the client.  If
your
first ca_put_callback() has not completed when the second one is
processed, it
will be rejected.  This requires that at least one of the records in the
processing chain be asynchronous though.
If the processing chain is all synchronous then the record processing all
takes place in the context of one of the RSRV (CA server) tasks.  In this
case
the putNotify completion will be called from the recGblFwdLink()
processing of
the last record in the chain, but still in the context of the RSRV thread,
so
the CA server can't start processing your second ca_put_callback() request
until the first one has finished and there's no conflict between your two
ca_put_callback() operations.
However that's not going to be true if there's any asynchronous processing
involved.  In that case the putNotify can still be active when the
process()
operation requested by RSRV thread returns, so the server can try
processing
the second ca_put_callback() operation but it may be rejected.
Of course if the CA server you're talking to is not an IOC database then
the
behaviour you get depends on capabilities of the underlying server tool,
which
might perversely object to some other interleaving that the IOC database
has
no problems with.
HTH,
- Andrew
--
If a man is offered a fact which goes against his instincts, he will
scrutinize it closely, and unless the evidence is overwhelming, he will
refuse to believe it.  If, on the other hand, he is offered something
which affords a reason for acting in accordance to his instincts, he
will accept it even on the slightest evidence.  -- Bertrand Russell

--
Tim Mooney ([email protected]) (630)252-5417
Software Services Group, Advanced Photon Source, Argonne National Lab.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System