Modify the data structures for multiple-mode locks and the algorithms for read_lock(X), write_lock(X), and unlock(X) so that upgrading and downgrading of locks are possible. (Hint: The lock needs to keep track of the transaction id(s) that hold the lock, if any.)

Question

What will be an ideal response?

Answer 1

We assume that a List of transaction ids that have read-locked an item is maintained, as
well as the (single) transaction id that has write-locked an item. Only read_lock and
write_lock are shown below.

```
read_lock (X, Tn):
B: if lock (X) = "unlocked"
then begin lock (X) <- "read_locked, List(Tn)";
no_of_reads (X) <- 1
end
else if lock(X) = "read_locked, List"
then begin
(* add Tn to the list of transactions that have read_lock on X *)
lock (X) <- "read_locked, Append(List,Tn)";
no_of_reads (X) <- no_of_reads (X) + 1
end
else if lock (X) = "write_locked, Tn"
(* downgrade the lock if write_lock on X is held by Tn itself *)
then begin lock (X) <- "read_locked, List(Tn)";
no_of_reads (X) <- 1
end
else begin
wait (until lock (X) = "unlocked" and
the lock manager wakes up the transaction);
goto B;
end;
write_lock (X,Tn);
B: if lock (X) = "unlocked"
then lock (X) <- "write_locked, Tn"
else
if ( (lock (X) = "read_locked, List") and (no_of_reads (X) = 1)
and (transaction in List = Tn) )
(* upgrade the lock if read_lock on X is held only by Tn itself *)
then lock (X) = "write_locked, Tn"
else begin
wait (until ( [ lock (X) = "unlocked" ] or
[ (lock (X) = "read_locked, List") and (no_of_reads (X) = 1)
and (transaction in List = Tn) ] ) and
the lock manager wakes up the transaction);
goto B;
end;
```