Re: back-bdb deadlocks?

[Howard Chu <hyc@symas.com>]

Howard Chu wrote:

> A read operation can cause an undetected deadlock when loading an entry 
> into the cache. This is because the lockerID used to lock the cache 
> entry is not the same as the lockerID BDB generates when accessing the 
> database. If a writer has already locked the id2entry pages and is 
> waiting on a write lock for the cache entry, then the reader will be 
> stuck waiting for the id2entry pages and neither will make any progress.
>
> This implies that we must use transactions for readers as well as 
> writers, to insure that all locks associated with a read operation get 
> the same lockerID, so that deadlocks can be detected by BDB. Either that 
> or we need to tweak the lock ordering again so that a reader cannot hold 
> any other locks while it is accessing the database.

I've checked in a fix for this. Creating a new transaction for every 
search operation would add a fair amount of overhead, so I've allocated 
long-lived per-thread transactions instead (similar to the way locker 
IDs are being re-used). We can't just replace the existing lockerID 
mechanism with these transactions, because the intended lifetimes of 
their locks are not the same. Anyway, I think this should finally 
resolve the deadlock issues.

Please test...
--
  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc
  Symas: Premier OpenSource Development and Support