Sql-server – Where should I fix the deadlock

Try turning on Read Committed Snapshot isolation (RCSI) (but be aware this will put increased pressure on your tempDB, which should ideally be on its own set of dedicated physical spindles).

There are two 'snapshot' levels available in SQL Server 2005 onwards: READ COMMITTED SNAPSHOT does optimistic reads and pessimistic writes; whereas SNAPSHOT ISOLATION performs optimistic reads and optimistic writes. Suggest you try RCSI.

Enabling Row Versioning-Based Isolation Levels

To alter this setting, you need to switch to single-user mode to ensure there are no queries in flight (which would then fail):

ALTER DATABASE dbname 
SET SINGLE_USER WITH ROLLBACK IMMEDIATE; 
GO

ALTER DATABASE dbname
SET READ_COMMITTED_SNAPSHOT ON; 
GO

ALTER DATABASE dbname
SET MULTI_USER; 
GO

The data you need is recorded in the default extended events trace.

DECLARE @xml XML

SELECT @xml = target_data
FROM   sys.dm_xe_session_targets
       JOIN sys.dm_xe_sessions
         ON event_session_address = address
WHERE  name = 'system_health'
       AND target_name = 'ring_buffer'

SELECT   
             XEventData.XEvent.query('(data/value/deadlock)[1]')  AS DeadlockGraph,
             CAST(XEventData.XEvent.value('(data/value)[1]', 'varchar(max)') AS XML) AS DeadlockGraph,
              XEventData.XEvent.value('(./@timestamp)[1]', 'DATETIME2') AS [DateTime]
FROM   (SELECT @xml AS TargetData) AS Data
       CROSS APPLY 
       TargetData.nodes ('RingBufferTarget/event[@name="xml_deadlock_report"]') AS XEventData (XEvent) 
ORDER BY [DateTime] DESC

Though it won't be there any more if you have restarted the service -e.g. to apply a trace flag or if the buffer has cycled in the meantime.

You can set up your own extended events trace that stores the deadlock graph to a file target for persistent non volatile storage. Example Code here. I personally find the deadlock graph XML more friendly than the trace flag output.

Edit

1. @MartinC points out in the comments that on instances of SQL Server that don't have all the updates there might be a problem with it generating invalid XML. The fix for that is to do some search and replace and use CAST(REPLACE(REPLACE(XEventData.XEvent.value('(data/value)[1]', 'varchar(max)'), '<victim-list>', '<deadlock><victim-list>'), '<process-list>', '</victim-list><process-list>') AS XML) AS DeadlockGraph in the SELECT list as described here.
2. Wayne Sheffield has posted a useful script to shred the deadlock graph XML into tabular format here.