On 07/05/15 8:13 am, Lidong Zhong wrote:

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On 5/5/2015 at 08:10 PM, in message [ref], Abhijit

Bhopatkar [off-list ref] wrote:

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On 05/05/15 3:14 pm, Abhijit Bhopatkar wrote:

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On 05/05/15 2:52 pm, Lidong Zhong wrote:

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On 5/1/2015 at 02:36 AM, in message [ref], Abhijit

Bhopatkar [off-list ref] wrote:

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To illustrate the problem consider timeline for two senders and one
receiver (we will ignore receive part for Sender2 node)

Sender1              Sender2                         Receiver
Get EX on TOKEN       Get EX on TOKEN
<Granted>                    <Wait till granted>

Get EX on MSG
write LVB
down MSG to CR
Get EX of ACK
<wait till granted>
        BAST for ACK
                                                               Get CR on MSG
                       read LVB
                       process
                       release ACK
AST for ACK
down ACK to CR
release MSG
release TOKEN
                      <granted>
                      Get EX on MSG

I am afraid this corner case could not be achieved ever. Sender2 will be

blocked on getting

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EX lock on MSG resource until the receivers release the lock. The

receivers' request on

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upconverting CR to EX on MSG should be put into the convert queue before

Sender2's

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request being put into the wait queue, because sender2 has to wait until

the EX on TOKEN

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is released.

Yes my initial though of losing a message is not correct. The EX on message

won't be granted

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immediately to Sender2 However there is still a deadlock.

Perhaps i am missing something, but according to me nothing prevents

Sender2 from acquiring

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EX on TOKEN _and_ MESSAGE __before__ up convert from reciever is queued.

Consider adding

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unusual delay right after ACK is released on receiver. The Sender1 will

immediately release

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MESSAGE and TOKEN. The receiver is still delayed for whatever reason.

Sender2 gets TOKEN grant

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and immediately queues EX for MESSAGE (note this is before EX for MESSAGE

is queued by receiver).

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Yes, there is a possibility leading to deadlock here.

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DLM will (should?) return error for the up convert saying there is deadlock

(-EDEADLK ??)

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On further investigation in dlm code. Since we do not set DLM_LKF_CONVDEADLK
flag on our locks,
in above deadlock case receiver's request to up convert will be simply
canceled. And the code
will proceed as expected since receiver still holds CR on MESSAGE. And then
after the processing
we will release the CR.

So now my question is changed to;

Why do we up convert the MESSAGE to EX in the first place?

Was receiver EX on MESSAGE intended to serialize all receivers before taking
CR on ACK?

Yes, it is. Otherwise, each receiver may get duplicate messages when they try to
get CR on ACK while the sender doesn't downconvert EX on ACK in time.

If I am reading this right, are we afraid of getting second BAST call on receiver?
Sender is holding EX on ACK, receiver releases CR of ACK after processing the message.
But sender is delayed in releasing EX on ACK. Receiver re-queues CR on ACK, which
might trigger BAST? (Note receiver won't get CR grant until sender released EX).

A new CR by receiver on ACK will _not_ trigger BAST call. Instead no AST will be called
until the original EX on ACK by sender is not released. BAST is called only on locks
that are already granted. Since we trigger message processing only on BAST I don't
see a possibility of duplicate message here.

What I can think of a way to fix the deadlock now is setting the DLM_LKF_NOQUEUE
flag when the sender tries to get EX on MESSAGE. It should keep trying until all the
receivers release their locks on MESSAGE. Do you have any better idea without adding
more lock resources? Since we already have three for transmitting messages.

Its exactly what I was thinking about and sounds like a good solution. However
as said  above I don't think receiver EX on ACK is really needed.

Regards,
Abhijit

Regards,
Lidong

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Since there is a possibility that we might lose out on this up convert in a
race  condition, can
we simply eliminate this up conversion? (since CR is preventing the next
Sender from taking
EX on MESSAGE anyway).

Regards,
Abhijit

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