Save conflict when adding two SPListItem simultaneously in ListItem Event Receiver

Question

Let's say I have two lists with following fields:

List A
------
+ Title
+ MultiLookupOnListB (Lookup List: List B; Lookup Field: Title)

List B
------
+ Title
+ LookupOnListA (Lookup List: List A; Lookup Field: Title)

So basically a List A Item can reference one or more List B Items while a List B Item can only reference one List A Item. I already set List B -> List A lookup with CSR, but the List A -> List B lookup needs to be done in an event receiver since I need access to resources that aren't available when using JS.

So I use ItemUpdating for adding List B items to List A multi lookup field. Only problem is, when two list items are created at the same time, the code is too fast (^^) and the second event basically overwrites the first event so I end up with only one referenced List B Item.

/// <summary>
/// An item was updated.
/// </summary>
public override void ItemUpdated(SPItemEventProperties properties)
{
    base.ItemUpdated(properties);
    var listItem = properties.ListItem;
    var listAItem = GetListAItemFromSomewhere();

    SPFieldLookupValueCollection listItemALookup = listAItem["MultiLookupOnListB "] as SPFieldLookupValueCollection;
    if (listItemALookup.OfType<SPFieldLookupValue>().Where(pdr => pdr.LookupId == listItem.ID).Count() < 1)
    {
        listItemALookup.Add(new SPFieldLookupValue(listItem.ID, listItem.Title));

        listAItem["MultiLookupOnListB "] = listItemALookup;
        listAItem.SystemUpdate(false);
    }
}

I have already tried to set the ItemUpdating event to synchronous, but that didn't work. So, any ideas on how to best do this?

Answer 1

Setting the event as synchronous cannot help indeed, since it means "blocking" with regards to the current request; it's not about blocking all requests in the execution pipe.
Your option here would be to use a real semaphore/locking system. That could be simple if you're in a single-server farm, as opposed to multi-server farm. What's your situation regarding your farm setup (I mean essentially: how many WFE servers)?

EDIT: farm-wide lock:

It's not simple to acquire a lock visible from all machines in the farm. One approach is to store the lock state in the SQL database.

https://stackoverflow.com/a/18738491 explains how to implement it in C#.
It will probably require some changes to adapt it to a SharePoint context: e.g. reverting to the process (app pool) identity before making the SQL calls, so you have the correct permissions (this can be done by issuing the commands from inside a using (HostingEnvironment.Impersonate()) section).