[asterisk-dev] [Code Review]: Resolve odbc segfaults by adding shared locks around usage of odbc handle in res_odbc

[Tilghman Lesher]

> On Feb. 8, 2012, 2:33 a.m., wdoekes wrote:
> > /branches/1.8/include/asterisk/res_odbc.h, line 160
> > <https://reviewboard.asterisk.org/r/1719/diff/1/?file=23900#file23900line160>
> >
> >     +1 on the _wrlocked suffix. It beats '2' any single day.
> 
> Tilghman Lesher wrote:
>     I don't really agree with that.  The problem comes to any new developer who sees a host of different APIs and wonders which one is the latest.  Adding a '2' makes it very clear that it's a later version of the function.  I'd also prefer if the underlying 2-argument function were exposed, instead of hiding it (and thus permitting new users to pass either 1 or 0 themselves), instead of doubling the number of APIs they need to remember.
> 
> wdoekes wrote:
>     If you put it like that, sure. But that only makes sense if your second preference is met; exposing the lock_type integer. And in this case, we'd have to know which integer means what (was 0 unlocked or readlocked?), which means adding defines or enums which we have to keep track of.
>     
>     Having said that, I wonder why the function is exposed at all. No one outside res_odbc is using it, except perhaps 3rd party callers? And I believe you already pointed out that we're already breaking things for them: they won't have the read-lock on it at all.
>

Well, that creates another problem entirely.  Currently, the way the code is written, sanity_check has no lock at all, and if we require a lock, then it should acquire it in the old API function.  Separately, we should be able to indicate (in the new API) that we already have a read lock, which may need to be escalated to a write lock (with an emitted warning), and finally, we should be able to indicate that we already have a write lock, which does not need to be escalated (and thus avoid an emitted warning).

I considered some of these issues when I wrote the code, which is why I used a mutex instead of a rwlock.  Rwlocks are great for concurrency, but they considerably complicate the code when APIs need the ability to escalate locks.  (Not to mention that if an API escalates to a write lock and returns to a caller that otherwise kept a persistent read lock, you have none of the advantages of having used rwlocks in the first place).


- Tilghman


-----------------------------------------------------------
This is an automatically generated e-mail. To reply, visit:
https://reviewboard.asterisk.org/r/1719/#review5432
-----------------------------------------------------------


On Feb. 8, 2012, 8:47 a.m., jrose wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviewboard.asterisk.org/r/1719/
> -----------------------------------------------------------
> 
> (Updated Feb. 8, 2012, 8:47 a.m.)
> 
> 
> Review request for Asterisk Developers, Mark Michelson, Tilghman Lesher, rmudgett, and wdoekes.
> 
> 
> Summary
> -------
> 
> This is a continuation of wdoekes patch seen in https://reviewboard.asterisk.org/r/1622/
> This adds Tilghman Lesher's suggested changes.
> 
> 
> This addresses bug ASTERISK-19011.
>     https://issues.asterisk.org/jira/browse/ASTERISK-19011
> 
> 
> Diffs
> -----
> 
>   /branches/1.8/res/res_odbc.c 354164 
>   /branches/1.8/include/asterisk/res_odbc.h 354164 
> 
> Diff: https://reviewboard.asterisk.org/r/1719/diff
> 
> 
> Testing
> -------
> 
> I started by confirming that the segfault could be easily reproduced with wdoekes' setup mentioned in the issue report.
> After finishing the changes to the patch, I confirmed that following the same steps no longer produced the segfault, undid the patch to double check that I was doing it right, and then decided it was safe to post this review.
> 
> 
> Thanks,
> 
> jrose
> 
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.digium.com/pipermail/asterisk-dev/attachments/20120208/74889a49/attachment.htm>