SharePoint Permissions GONE WILD!
Written By: Joshua Fuente -- 5/18/2011 --
(5) comments --
Categories: Configurations, Infrastructure, MOSS 2007, Permission Management, System Administration, WSS3
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You have a mature SharePoint 2007 installation, but lets say
you have not been able to update it... And then one day, you get "the
call". (And the sky is falling, of course!)...
...The Helpdesk has just informed you that all users are getting
the following error when attempting to access one or more site collections.
"Attempt to release mutex not owned by caller. (Exception from
Description: An unhandled exception occurred during the
execution of the current web request. Please review the stack trace for more
information about the error and where it originated in the code.
Exception Details: System.ApplicationException: Attempt to
release mutex not owned by caller. (Exception from HRESULT: 0x00000120)"
While there could be many
reasons for this type of exception. We are dealing with SharePoint, so all bets are
off. :) So you login and take a look. And sure enough, one or more
of your larger site collections is giving this error. So you being the
super sleuth SharePoint admin, pull out trick number one! You access the
site collection settings page, (/_layouts/setting.aspx). And oddly enough,
it appears, no problem, no error. You can access most settings, and even some
pages in the pages library. (This rules out my initial "guess" that
this might be SQL related.) Everything else seems to work, but wait, the master
page settings page is throwing an error too! Claims the master page is
invalid...? huh? How would that happen all of a sudden? So what's the common
What you most likely find is that its the cached pages giving the error...
The Output Cache to be exact. It has what I would consider to be, an
undocumented limit, not posted in the
software boundaries page on TechNet. Go figure... So ok, what is it
then? Why this site? Why now? More importantly, how do I
The real root cause is that Output Cache will only support 10,000
Access Control Lists (ACLs). And how do you end up with 10k ACLs? You most likely have a large site collection, with a lot of unique permissions,
with users breaking inheritance at the item or folder level. And then there's
the ability to add "All Authenticated Users" and to a SharePoint
group, or even direct permission to the site, web, or item. You also have
to consider users that have been disabled in your profile import source, but
have not been removed from your site collection. You add that all up, and
you can hit 10k very quickly. And I cannot emphasize enough that 10k is
the limit. I have seen the query I am going to show, you return as few as
10042, on a afflicted site collection. So it's pretty cut and dry, you
have to clean up permissions, or turn off Output Cache. In most cases, you
will simply turn off Output Cache while the "content/site-collection
administrators" clean up the permissions, and then re-enable Output Cache.
Now for the icing on the cake, the
April 2009 Cumulative update
for SharePoint 2007, does take a step to address the issue. Albeit,
a superficial one... What the afore mentioned update does, is simply avoid
throwing the exception. When the number of ACLs goes over 10,000, Output Cache is still disabled, it just does not produce the error. So how then
would you know about the problem? Well, you may start getting complaints
about performance... But you will probably only know if you check. Yup, no
one else is going to do it are they? Lucky, there' s an easy way to see if
your Output Cache is working. In a future post, I will walk through some of the
ways you can test output and the BLOB cache.
The Output Cache is not on by default, but most SharePoint administrators,
particularly at the enterprise level, are keenly aware of it, and most of them
recommend it's use. And I have to say, this is the first time I have seen
it go afoul. What the Output Cache is best at and best used for; is loading items
into memory that are frequently "read" but infrequently "modified". Portal
landing pages are typically a good example, lots of people view them, but
changes are usually not all that frequent.
For more information about the Output Cache, see the Microsoft documentation;
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will find a note about the issue here, but not in the software boundaries guide.