[asterisk-dev] [Code Review] 4188: segfault when playing back voicemail under high concurrency with an IMAP backend

[Matt Jordan]

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>From http://www.washington.edu/imap/documentation/internal.txt.html:

{quote}
			* * * IMPORTANT * * *

     Any multi-threaded application should test stream->lock prior to
calling any c-client stream functions.  Any attempt to call a
mail_xxx() function while one is already in progress on the same
stream will cause the application to fail in unpredictable ways.

     Note that this check is insufficient in a preemptive-scheduling
multi-tasking application due to the possibility of a timing race.
Such applications must be written so that only one process accesses
the stream, or to have a higher level lock.

     Since MAIL operations will not finish until they are completed, a
single-tasking application does not have to worry about this problem,
except in the callback invoked from MAIL (e.g. mm_exists(), etc.) in which
case the stream is *always* locked.
{quote}

So, a few things from this:
(1) Based on the description from the UW IMAP documentation, locking vms->lock is sufficient *if* other mail accesses also lock the same vms object for that stream. If that isn't happening, then your patch probably won't fix anything, since both mail_open calls were already protected by the vms->lock and your global lock doesn't protect any other mail_XXXX accesses. The only way your patch would affect anything is if there were two different vms objects being used to open the same stream at the same time.
(2) Your backtrace on the issue doesn't show concurrent accesses to vm_execmain - although a lot of symbols are missing - so right now it's impossible to tell who the offending accesses are. It's also impossible to know if it is due to two concurrent accesses of mail_open with different vms objects, or something else.

Generally, I'm not this patch fixes your issue.

- Matt Jordan


On Nov. 17, 2014, 9:03 a.m., David Duncan Ross Palmer wrote:
> 
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> This is an automatically generated e-mail. To reply, visit:
> https://reviewboard.asterisk.org/r/4188/
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> 
> (Updated Nov. 17, 2014, 9:03 a.m.)
> 
> 
> Review request for Asterisk Developers.
> 
> 
> Bugs: ASTERISK-24516
>     https://issues.asterisk.org/jira/browse/ASTERISK-24516
> 
> 
> Repository: Asterisk
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> Description
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> Asterisk segfaults when playing back voicemail under high concurrency with an IMAP backend
> 
> 
> Diffs
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> 
>   /trunk/apps/app_voicemail.c 427675 
> 
> Diff: https://reviewboard.asterisk.org/r/4188/diff/
> 
> 
> Testing
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> Rapid load testing performed with {{SIPp}}:
> {code}
> sipp -sn uac -d 7000 -s ‘*1’ 127.0.0.1 -l 400 -mp 5606
> {code}
> 
> 
> Thanks,
> 
> David Duncan Ross Palmer
> 
>

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