Re: Potential race in dlm based messaging md-cluster.c

[Abhijit Bhopatkar <abhopatk@cisco.com>]

On 01/05/15 12:06 am, Abhijit Bhopatkar wrote:
> There is a possibility of a receiver losing out on messages in certain
> corner conditions. One of the buggy case is if there is are two sender
> ready with messages to be sent. Sender 1 initially gets the TOKEN lock
> and proceeds.
> After initial processing the sender of message 1 _will_ release TOKEN as
> soon as receiver releases ACK, it does not wait till ACK CR is
> re-acquired by receiver.
>
I could not come up with any solution except to add one more lock
resource for now we will call it "SYNC"

Sender 1             Sender2                  Receiver
Get EX on TOKEN      Get EX on TOKEN
Get EX on SYNC       <Wait till granted>
<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>
                                               Queue EX on SYNC
                                               release ACK
AST for ACK
down ACK to CR
release MSG
release SYNC
release TOKEN
                                                SYNC  granted
                     <granted>
                     Get EX on SYNC
                     <wait till grant>
                                                Get EX on MSG
                                                Get CR on ACK
                                                release MSG
                                                release SYNC

                     Get EX on MSG
                     <....proceed rest>
                     release TOKEN

The key thing to note here is that the SYNC lock request is only queued
in receiver path. Having worked in dlm before I know for sure this will
work as expected.

Abhijit