[asterisk-dev] [Code Review] 2961: Add channel locking for calls that result in channel snapshot creation

[Mark Michelson]

> On Oct. 29, 2013, 11:08 p.m., rmudgett wrote:
> > /branches/12/main/core_local.c, lines 378-380
> > <https://reviewboard.asterisk.org/r/2961/diff/1/?file=47579#file47579line378>
> >
> >     Uh oh.  Locking this channel is dangerous.  At this point, we already have two channels, two bridge_channels, and two bridges locked.

Of the issues raised, this is the one that I'm least sure of how to proceed on.

One possible solution would be to change the optimization_started callback to return an int instead of void. Here in core_local.c, if an ast_channel_trylock() of source fails, then we can return failure, thus making the move-swap local optimization fail until the next attempt is made. What do you think of that?


- Mark


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On Oct. 25, 2013, 5:07 p.m., Mark Michelson wrote:
> 
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> This is an automatically generated e-mail. To reply, visit:
> https://reviewboard.asterisk.org/r/2961/
> -----------------------------------------------------------
> 
> (Updated Oct. 25, 2013, 5:07 p.m.)
> 
> 
> Review request for Asterisk Developers.
> 
> 
> Bugs: ASTERISK-22709
>     https://issues.asterisk.org/jira/browse/ASTERISK-22709
> 
> 
> Repository: Asterisk
> 
> 
> Description
> -------
> 
> John Bigelow discovered that if the newly-created atxfer_threeway_nominal test is run in a loop, eventually a test run will cause the UUT instance of Asterisk to crash. Investigation of the crash showed that the crash was being caused while trying to get the string representation of a channel's translation paths while creating a channel snapshot. The attended transfer manager thread was creating the channel snapshot as part of the attended transfer stasis message while in another thread (in this case, the bridge channel thread) the translation paths were being altered to make the channel compatible with others in the bridge.
> 
> It's clear that the issue is that the channel's contents are not lock protected while creating a channel snapshot. Luckily, the channel's lock is held already when updating translation paths. By adding locking, I was able to run the test over 450 times straight with no negative consequences.
> 
> When adding locking, there were two strategies to follow:
> 
> 1) Take advantage of the fact that channel locks are recursive, and just add a single SCOPED_CHANNELLOCK() call to ast_channel_snapshot_create() and call it a day.
> 2) Make an audit of channel-related calls that end up creating a channel snapshot and require locks be held prior to making those calls.
> 
> I like approach 2 more, even though it's harder to pull off. In investigating what paths result in channel snapshots being created, I found that channel locks should be required before calling the following functions:
> 
> * ast_channel_snapshot_create()
> * ast_channel_blob_create()
> * ast_channel_publish_snapshot()
> * ast_publish_channel_state()
> * ast_channel_publish_blob()
> * ast_channel_publish_varset()
> * ast_channel_amaflags_set()
> * ast_channel_callid_set()
> * ast_channel_whentohangup_set()
> * ast_channel_callgroup_set()
> * ast_channel_pickupgroup_set()
> * ast_channel_internal_bridge_set()
> * ast_channel_language_set()
> * ast_channel_accountcode_set()
> * ast_channel_peeraccount_set()
> * ast_channel_linkedid_set()
> * ast_channel_stage_snapshot()
> * ast_setstate()
> * ast_aoc_manager_event()
> * ast_channel_setwhentohangup_tv()
> * ast_channel_setwhentohangup()
> * ast_set_variables()
> 
> All of these functions' documentation have been updated to indicate a precondition of having the channel locked.
> 
> In addition, there are some more complex functions that create channel snapshots but could not easily be made to have the precondition of having the channel locked. These are:
> 
> * ast_bridge_blob_create()
> * ast_publish_attended_transfer_fail()
> * ast_publish_attended_transfer_bridge_merge()
> * ast_publish_attended_transfer_threeway()
> * ast_publish_attended_transfer_app()
> * ast_publish_attended_transfer_link()
> * ast_bridge_publish_enter()
> * ast_bridge_publish_leave()
> * ast_bridge_publish_blind_transfer()
> 
> These functions' documentation now have a precondition of having no channels or bridges locked.
> 
> This review seeks only to make sure that channels are properly locked prior to creating channel snapshots. A similar effort is likely needed to ensure that bridges are locked during bridge snapshot creation.
> 
> 
> Diffs
> -----
> 
>   /branches/12/tests/test_stasis_channels.c 401703 
>   /branches/12/tests/test_cel.c 401703 
>   /branches/12/tests/test_cdr.c 401703 
>   /branches/12/res/res_stasis.c 401703 
>   /branches/12/res/res_pjsip_refer.c 401703 
>   /branches/12/res/res_agi.c 401703 
>   /branches/12/res/parking/parking_manager.c 401703 
>   /branches/12/res/parking/parking_bridge_features.c 401703 
>   /branches/12/res/ari/resource_bridges.c 401703 
>   /branches/12/pbx/pbx_realtime.c 401703 
>   /branches/12/main/stasis_channels.c 401703 
>   /branches/12/main/stasis_bridges.c 401703 
>   /branches/12/main/pickup.c 401703 
>   /branches/12/main/pbx.c 401703 
>   /branches/12/main/endpoints.c 401703 
>   /branches/12/main/dial.c 401703 
>   /branches/12/main/core_unreal.c 401703 
>   /branches/12/main/core_local.c 401703 
>   /branches/12/main/channel.c 401703 
>   /branches/12/main/cel.c 401703 
>   /branches/12/main/bridge_channel.c 401703 
>   /branches/12/main/bridge.c 401703 
>   /branches/12/include/asterisk/stasis_channels.h 401703 
>   /branches/12/include/asterisk/stasis_bridges.h 401703 
>   /branches/12/include/asterisk/channelstate.h 401703 
>   /branches/12/include/asterisk/channel.h 401703 
>   /branches/12/include/asterisk/aoc.h 401703 
>   /branches/12/funcs/func_timeout.c 401703 
>   /branches/12/channels/sig_pri.c 401703 
>   /branches/12/channels/sig_analog.c 401703 
>   /branches/12/channels/chan_vpb.cc 401703 
>   /branches/12/channels/chan_unistim.c 401703 
>   /branches/12/channels/chan_skinny.c 401703 
>   /branches/12/channels/chan_sip.c 401703 
>   /branches/12/channels/chan_pjsip.c 401703 
>   /branches/12/channels/chan_phone.c 401703 
>   /branches/12/channels/chan_oss.c 401703 
>   /branches/12/channels/chan_nbs.c 401703 
>   /branches/12/channels/chan_motif.c 401703 
>   /branches/12/channels/chan_misdn.c 401703 
>   /branches/12/channels/chan_mgcp.c 401703 
>   /branches/12/channels/chan_jingle.c 401703 
>   /branches/12/channels/chan_iax2.c 401703 
>   /branches/12/channels/chan_h323.c 401703 
>   /branches/12/channels/chan_gtalk.c 401703 
>   /branches/12/channels/chan_dahdi.c 401703 
>   /branches/12/channels/chan_console.c 401703 
>   /branches/12/channels/chan_alsa.c 401703 
>   /branches/12/apps/app_voicemail.c 401703 
>   /branches/12/apps/app_userevent.c 401703 
>   /branches/12/apps/app_queue.c 401703 
>   /branches/12/apps/app_meetme.c 401703 
>   /branches/12/apps/app_disa.c 401703 
>   /branches/12/apps/app_dial.c 401703 
>   /branches/12/apps/app_confbridge.c 401703 
>   /branches/12/apps/app_agent_pool.c 401703 
>   /branches/12/addons/chan_ooh323.c 401703 
>   /branches/12/addons/chan_mobile.c 401703 
> 
> Diff: https://reviewboard.asterisk.org/r/2961/diff/
> 
> 
> Testing
> -------
> 
> As stated in the description, I ran the atxfer_threeway_nominal test in a loop. After approximately 450 test runs, there were no negative consequences. Prior to this changeset, running the test 20-50 times would result in a crash.
> 
> 
> Thanks,
> 
> Mark Michelson
> 
>

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