Deadlock executing Query in Sql Server

Question

I've the following Linq:

var qry = s.GetTable<MessageEventDTO>().Where(x => x.MessageName == messageName && x.SourceTyp == sourceTyp && x.Source == source && (x.Status == MessageEventStatus.open || x.Status == MessageEventStatus.acknowledged));

goneMessages = qry.ToList();

var ret = qry
    .Set(x => x.Status, x => x.Status | MessageEventStatus.gone)
    .Set(x => x.TimestampGone, timeStamp)
    .Update();
return ret;

which will be converted to following SQL:

UPDATE MessageEvents SET Status = Status | 1, TimeStampGone = @1
WHERE MessageName = @2 AND SourceTyp = @3 Source = @4 AND (Status = 0 OR Status = 2)

the problem is now, there are multiple Updates run in parallel, and I got deadlock exceptions, but I do not understand why?

see also

Answer 1

If you don't want another process to be able to start the select portion of the update, use the UPDLOCK hint or set an appropriate transaction isolation level ( REPEATABLE READ ).

See John Huang's Blog for a more thorough explanation of how nonclustered indexes can cause deadlocks.

Example using Linq to SQL which is not prey to this issue:

var opts = new TransactionOptions();
opts.IsolationLevel = IsolationLevel.RepeatableRead;
using (var txn = new TransactionScope(TransactionScopeOption.Required, opts))
{
    // update command goes here.

    txn.Complete();
}