On 05/05/15 3:14 pm, Abhijit Bhopatkar wrote:
On 05/05/15 2:52 pm, Lidong Zhong wrote:
On 5/1/2015 at 02:36 AM, in message <5542763C.90202@xxxxxxxxx>, Abhijit
Bhopatkar <abhopatk@xxxxxxxxx> wrote:
<snip>
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
EX lock on MSG resource until the receivers release the lock. The receivers' request on
upconverting CR to EX on MSG should be put into the convert queue before Sender2's
request being put into the wait queue, because sender2 has to wait until the EX on TOKEN
is released.
Yes my initial though of losing a message is not correct. The EX on message won't be granted
immediately to Sender2 However there is still a deadlock.
Perhaps i am missing something, but according to me nothing prevents Sender2 from acquiring
EX on TOKEN _and_ MESSAGE __before__ up convert from reciever is queued. Consider adding
unusual delay right after ACK is released on receiver. The Sender1 will immediately release
MESSAGE and TOKEN. The receiver is still delayed for whatever reason. Sender2 gets TOKEN grant
and immediately queues EX for MESSAGE (note this is before EX for MESSAGE is queued by receiver).
DLM will (should?) return error for the up convert saying there is deadlock (-EDEADLK ??)
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?
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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