[Asterisk-code-review] pbx: Deadlock between contexts container and context merge l... (asterisk[11])

[Kevin Harwell]

Hello Anonymous Coward #1000019, Matt Jordan, Joshua Colp,

I'd like you to reexamine a change.  Please visit

    https://gerrit.asterisk.org/1855

to look at the new patch set (#3).

Change subject: pbx: Deadlock between contexts container and context_merge locks
......................................................................

pbx: Deadlock between contexts container and context_merge locks

Recent changes (ASTERISK-25394 commit 2bd27d12223fe33b58c453965ed5c6ed3af7c4f5)
introduced the possibility of a deadlock. Due to the mentioned modifications
ast_change_hints now needs to keep both merge/delete and state callbacks from
occurring while it executes. Unfortunately, sometimes ast_change_hints can be
called with the contexts container locked. When this happens it's possible for
another thread to grab the context_merge_lock before the thread calling into
ast_change_hints does and then try to obtain the contexts container lock. This
of course causes a deadlock between the two threads. The thread calling into
ast_change_hints waits for the other thread to release context_merge_lock and
the other thread is waiting on that one to release the contexts container lock.

Unfortunately, there is not a great way to fix this problem. When hints change,
the subsequent state callbacks cannot run at the same time as a merge/delete,
nor when the usual state callbacks do. This patch alleviates the problem by
having those particular callbacks (the ones run after a hint change) occur in a
serialized task. By moving the context_merge_lock to a task it can now safely be
attempted or held without a deadlock occurring.

ASTERISK-25640 #close
Reported by: Krzysztof Trempala

Change-Id: I7f67ce4b4c94b731c23932cc2ba9e0a4a720813d
---
M include/asterisk/event_defs.h
M main/pbx.c
2 files changed, 333 insertions(+), 311 deletions(-)


  git pull ssh://gerrit.asterisk.org:29418/asterisk refs/changes/55/1855/3
-- 
To view, visit https://gerrit.asterisk.org/1855
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Gerrit-MessageType: newpatchset
Gerrit-Change-Id: I7f67ce4b4c94b731c23932cc2ba9e0a4a720813d
Gerrit-PatchSet: 3
Gerrit-Project: asterisk
Gerrit-Branch: 11
Gerrit-Owner: Kevin Harwell <kharwell at digium.com>
Gerrit-Reviewer: Anonymous Coward #1000019
Gerrit-Reviewer: Joshua Colp <jcolp at digium.com>
Gerrit-Reviewer: Kevin Harwell <kharwell at digium.com>
Gerrit-Reviewer: Matt Jordan <mjordan at digium.com>