[asterisk-dev] [Code Review] 2915: Add channel lock protection around translation path setup.

[rmudgett]

> On Oct. 16, 2013, 9:18 p.m., Mark Michelson wrote:
> > I made a small effort to look at whether this added locking might cause issues. First file I checked was res_fax.c. If you look at the fax_gateway_framehook(), you'll find that chan is locked, and then formats are set on both the chan and peer channels (line ~3113 in the 12 branch). Setting the read and write formats on the peer at this point now introduces a potential deadlock.
> > 
> > I think that an audit of the potential effects of this change is required. One of two things needs to be done based on this audit.
> > 
> > 1) If it turns out I was unlucky and discovered the one and only potential deadlock spot in the code, or if there are only a couple of places like it, then those places need to be rearranged to prevent a potential deadlock.
> > 2) If this is a more widespread a problem, then you should not embed the locking down in the channel.c code but rather require that callers of the functions grab the lock themselves before calling.
> > 
> > Whichever way this goes, the affected public functions in channel.h need to be documented as either requiring a lock be grabbed before calling or as grabbing locks itself so that the implications are clear.

I fixed the deadlock potential you found in res_fax.c dealing with ast_channel_make_compatible().  The deadlock potential was already there anyway because res_fax called ast_channel_make_compatible() with chan locked.  Unfortunately, the res_fax framehooks have even more pre-existing deadlock potential remaining when there is a Local channel involved.


- rmudgett


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On Oct. 16, 2013, 3:58 p.m., rmudgett wrote:
> 
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> https://reviewboard.asterisk.org/r/2915/
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> 
> (Updated Oct. 16, 2013, 3:58 p.m.)
> 
> 
> Review request for Asterisk Developers.
> 
> 
> Bugs: ASTERISK-22542
>     https://issues.asterisk.org/jira/browse/ASTERISK-22542
> 
> 
> Repository: Asterisk
> 
> 
> Description
> -------
> 
> Most callers of ast_channel_make_compatible() happen before the channels enter a two party bridge.  With the new bridging framework, two party bridging technologies may also call ast_channel_make_compatible() when there is more than one thread involved with the two channels.
> 
> * Added channel lock protection in set_format() and ast_channel_make_compatible_helper() when dealing with the channel's native formats while setting up a translation path.
> 
> * Fixed best_src_fmt and best_dst_fmt usage consistency in ast_channel_make_compatible_helper().  The call to ast_translator_best_choice() got them backwards.
> 
> * Updated some callers of ast_channel_make_compatible() and the function documentation.  There is actually a difference between the two channels passed in.
> 
> 
> Diffs
> -----
> 
>   /branches/12/apps/app_dial.c 401095 
>   /branches/12/include/asterisk/channel.h 401095 
>   /branches/12/main/channel.c 401095 
> 
> Diff: https://reviewboard.asterisk.org/r/2915/diff/
> 
> 
> Testing
> -------
> 
> Since there is not enough information on ASTERISK-22542 to determine why no translation path could happen, the best that testing can do is show that the change does not cause problems.
> 
> 
> Thanks,
> 
> rmudgett
> 
>

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