How can I debug SessionStateModule/REQUEST_AQUIRE_STATE taking > 100 seconds on half of my requests?

Question

This started happening in the last few days; possibly since I installed Visual Studio 2012. It's currently only happening on my machine (my colleagues are all still on VS 2010), but I'm hoping to uninstall VS 2012 to see if that has an affect.

Around 50% of all my web requests hang in SessionStateModule/REQUEST_AQUIRE_STATE for around 100 seconds (then the page completes successfully).

ANTS Profiler reports all the time as "Awaiting synchronisation" and gives a stack trace in some IIS thread callback (none of our code; sadly I don't have the info to hand).

I enabled IIS Failed Request Tracing for all requests, and reloaded the same page of our app 10 times. Requests 3, 5, 7, 9 all took > 100 seconds, with the others taking < 5 seconds.

We're using In Process sessions, so nothing is going over the network or being serialised. As far as I can tell, none of our code has been run up until the point this happens.

How can I get more information on what's happening to debug the issue?

Possibly related:

Browser waits for ajax call to complete even after abort has been called (jQuery)

Answer 1

This is a bug in .NET 4.5; see "Issue 6" on this page:

http://support.microsoft.com/kb/2828841/en-us