How to replicate a SCH_S, SCH_M deadlock

Question

I have a large migration script (about 2000 lines) that fails because of a deadlock on some metadata, and the lock types are SCH_S and SCH_M according to some xEvents data I

Answer 1

Can someone help me recreate this kind of scenario in single process?

There is a simple demo of this issue here.

BEGIN TRAN

go

CREATE TYPE dbo.OptionIDs AS TABLE( OptionID INT PRIMARY KEY )

go

DECLARE @OptionIDs dbo.OptionIDs;

go

ROLLBACK 

It is certainly unusual to have a deadlock involving a single process and single resource.

The resource is metadatalock subresource="USER_TYPE" classid="user_type_id = 264"

The session already holds a SCH-M metadata lock on a user defined type (probably as you created it earlier in the script) and this blocks an attempt to get a SCH-S lock on the same object by the same session later.

The transaction name shown in the deadlock graph is @OptionIDs - this is an internal system transaction that writes to the tempdb transaction log when creating the instance of the table type corresponding with the DECLARE @OptionIDs above. This is a separate transaction from the surrounding user transaction which is why the session is unexpectedly unable to get a SCH-S lock despite already holding the SCH-M lock.

This is blogged about by Aaron Bertrand here