Seems similar to ZBX-2494 and related issues.
I've been busy with other things here, but I remember that I tracked this down to the update interval for low level discovery that I had configured. I think the interval that I set was too low and the previous discovery hadn't finish before a new one had begun.
I had set it to 60 seconds(to get discovery started), but then forgot to change it back. I haven't run into it since.
To note: I didn't know there was a separate discovery update interval parameter and it may be useful to note that this parameter shouldn't be set too low.
I'm not sure what caused the dead lock, but I suspect discovery was running while the items that where discovered previously, where being updated by their individual update interval.
I should also note that I'm now running pre-zabbix-1.9.6.x-21167.tar.gz, so it might have been fixed previously. It could also be that I pay more attention to the low level discovery update interval.
What would happen if the update interval on discovery was set so low that discovery would start before a previous run of discovery had ended?
just to clarify, you don't have "normal" network discovery, just lld - and the lld interval is the one you set to 60 seconds initially ?
> What would happen if the update interval on discovery was set so low that discovery would start before a previous run of discovery had ended?
low level discovery or network discovery ?
network discovery would start "that interval" after the previous one.
lld... after some clarification, it would run as normal, but all the incoming data would be processed sequentially and values from same lld rule would never be processed at the same time (unless there's a Terrible Bug somewhere 
)
>just to clarify, you don't have "normal" network discovery, just lld - and the lld interval is the one you set to 60 seconds initially ?
if my memory serves me correctly this would have been "normal" network discovery and during "normal" network discovery a lld template was being linked to hosts via a discovery action.
From ZBX-3770, >if you have network discovery that links to a template with lld, sometimes items get assigned to the wrong host ? i assume this happens only if network discovery finds more than one host ? ---Q1 is yes, I'm not exactly certain if the items get assigned incorrectly with any pedictablity though, I'd have to test it. Q2. is yes, a discovery action that links a template with lld works flawlessly one only one host is discovered
With that being said, perhaps having items linked via lld to the wrong host could cause the Lock wait timeout...
Fixed in the development branch svn://svn.zabbix.com/branches/dev/ZBX-3771
(1) Please review my changes in r22746.
<sasha> CLOSED
<asaveljevs> In some queries item_discovery table is aliases to "id", in others - just "d".
<sasha> RESOLVED
<asaveljevs> CLOSED
(2) Currently, when creating real items from item prototypes, SNMP community and other SNMP parameters are taken from discovery rules, even though it is possible to edit them for prototypes. SNMP parameters should be taken from item prototypes.
<sasha> RESOLVED
<asaveljevs> CLOSED
(3) Let's move the code for low-level discovery into a different module: src/libs/zbxdbhigh/proxy.c is not the best place for it.
<sasha> RESOLVED
<asaveljevs> Something tells me that leaving out file lld.c from the repository was not intended. 
<sasha> sorry, already added
<asaveljevs> CLOSED. I have only removed a trailing empty line from that file.
Available in version pre1.9.8, r23473.