RedisSessionStateProvider Elasticache deadlock - asp.net

Question

we want to share ASP.NET Session state between our apps and services. We chose Elasticache/redis to achieve this. It was going well but we've run into a deadlock scenario.
Here's the deadlock sequence:
user navigates to page served by App 1
App 1 uses RedisSessionStateProvider, successfully fetches the Session in a few milliseconds
App 1 makes an HttpWebRequest to App 2, with the ASP.NET_SessionId cookie attached
App 2 also uses RedisSessionStateProvider, which attempts to fetch the Session from the same redis instance and times-out after ~ 2 minutes
Presumably App 1's RedisSessionStateProvider is holding a (write?) lock on the cache item containing the Session. As you can tell from my parlance, I'm no redis guru...
AFAICT Elasticache gives you no visibility onto situations like this, just performance-y graphs. And RedisSessionStateProvider is closed-source so I can't poke around there.
I also tried to get RedisSessionStateProvider to log (via the loggingClassName parameter) but nothing gets written by either App 1 or App 2 (my Log() method is called though).
To prove that it is the RedisSessionStateProvider deadlocking (rather than our own code deadlocking) I switched App 1 back to using InProc sessions and everything runs fine.
Does anyone have any suggestions? BTW our Session data is to all intents and purposes immutable, so there really is no need for it to be locked.
Many thanks,
Pete
EDIT: the sessionState config as requested. Note that the large operationTimeoutInMilliseconds value is so that we don't get exceptions whilst debugging the app. This will be changed to ~ 5000 in production.
<sessionState mode="Custom" customProvider="RedisSessionProvider">
<providers>
<add name="RedisSessionProvider"
type="Microsoft.Web.Redis.RedisSessionStateProvider"
host = "ec2-184-73-3-249.compute-1.amazonaws.com"
port = "6379"
ssl = "false"
throwOnError = "true"
retryTimeoutInMilliseconds = "2000"
applicationName = "PE"
connectionTimeoutInMilliseconds = "2000"
operationTimeoutInMilliseconds = "1800000"
</providers>
</sessionState>

Answer 1

This is not answer but it is not fitting in comment section.
At start of page asp.net page execution life cycle it calls GetItemExclusive which fetches session from store (in this case redis) and puts a lock on that session so other parallel request cannot modify session while this request is working. This lock has time out that is equivalent of request timeout that you can set using web.config like below.
<configuration>
<system.web>
<httpRuntime executionTimeout="10"/>
</system.web>
</configuration>
Now, page executes and depending on weather anything was modified or not modified in session it calls SetAndReleaseItemExclusive or ReleaseItemExclusive which releases the lock. If this request fails for some reason than it will retry depending on retryTimeoutInMilliseconds value. If retryTimeoutInMilliseconds is very less or same as operationTimeoutInMilliseconds then it might not retry at all. If SetAndReleaseItemExclusive or ReleaseItemExclusive is not completed successfully then basically your session will be locked for complete time of “executionTimeout” that you have set above which is in seconds. All other request will be blocked and won’t be able to access the session while it is locked. Lock will be released automatically when it reaches expiry.
Use web.config properties loggingClassName and loggingMethodName for configuring logging. You can find more details in web.config comments when you upgrade to above package. You can basically provide a public, static method which returns a TextWriter. Session state provider and StackExchange.Redis.StrongName both will use this TextWriter object to log details.
This will help us get more details about problem. Beware that enabling logging will reduce the performance.
Example of using logging:
namespace SSPWebAppLatest3
{
public static class Logger
{
public static TextWriter GetLogger()
{
return File.CreateText("C:\\Logger.txt");
}
}
}
Web.config:
<add name="MySessionStateStore" type="Microsoft.Web.Redis.RedisSessionStateProvider" host="127.0.0.1" accessKey="" ssl="false"
loggingClassName="Logger, SSPWebAppLatest3, Version=1.0.0.0, Culture=neutral ……."
loggingMethodName="GetLogger"/>
Please send me a reproducible test app with which I debug this further. You can also do the same as session state and output cache provider code is open source now. (https://github.com/Azure/aspnet-redis-providers)