Re: Writable lock nested in read-only lock

Brian,

Brian Smith wrote:
> Hi,
> 
> I am writing some code like this:
> 
> repository.beginTrans(false);
> try {
>      // [1] read only part
>      boolean ok = false;
> 
>      if (I need to write to the repository then) {
>           repository.beginTrans(true);
>           try {
>               // [2] read-write part
>              ok = true;
>           } finally {
>              repository.endTrans(!ok);
>           }
>      }
>      // [3] read only part
> } finally {
>     repository.endTrans();
> }
> 
> As you can see, I have a write transaction nested inside of a read 
> transaction. The reason I am doing this is because I want to lock the 
> repository against _writes_ for all three parts, [1], [2], [3], but I 
> want to lock the repository against _reads_ for as little time as 
> possible [2]. However, MDR responds with an error: Writable lock nested 
> in read-only lock.
> 
> 
> Would it be possible to change MDR's behavior so that writable locks 
> could be nested inside read-only locks? Is this an unsound thing to do? 
> It seems sound to me (the inner call to beginTrans() would block until 
> all _other_ readers ended their transactions, just like any other call 
> to beginTrans(true)). Is there a better way to do what I want?
> 

You risk that when your thread will attempt to promote its read lock to 
a write one it will block (since there will be several other readers), 
possibly holding other resources locked (those it locked from [1] 
forward). If there's another thread, which also got the read lock and 
waits on that resource or will wait to the previous one's output, you 
get a deadlock.

-Svata

-- 
Svatopluk Dedic            <mailto:[email protected]>
NetBeans, Java/Repository  <http://java.netbeans.org>