Second transaction remains waiting after the first completes

Question

Running this same script in 2 windows in SSMS one after another with a couple of seconds' delay.
The second instance that I start later completes but the first one remains locked till I close the second window. What is happening here? Why is it not releasing the lock after the COMMIT?

SET TRAN ISOLATION LEVEL SERIALIZABLE
BEGIN TRAN

SELECT * FROM dbo.t1
WHERE id IN (1,3)

WAITFOR DELAY '00:00:20'

UPDATE t1 SET InUse=0
WHERE id IN (1,3)

COMMIT

Edit:

The table structure is as follows:

CREATE TABLE [dbo].[t1](
    [id] [INT] NOT NULL,
    [InUse] [BIT] NOT NULL DEFAULT ((0)),
PRIMARY KEY CLUSTERED 
(
    [id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

Answer 1

You should update your question with the table structure.

As you did not specify whether your table has any index on it I assume there hasn't, ie dbo.t1 is a heap .

In this case you'll get classic deadlock :

session1 wants to select from table and needs S for any row it reads and because there is no index on dbo.t1 the whole table is read and S-lock on the table is held for the whole duration of the transaction.

In meanwhile session2 does the same and it also acquires S-lock on the table and holds it.

Session1 now needs to convert its S-lock to IX in order to do update and it is blocked by session2 that holds S on dbo.t1 .

When session2 tries to do the same it leads to deadlock because both sessions need IX and both are locked by other session.

Here is the corresponding deadlock graph:

deadlock-list
 deadlock victim=process155d1d498
  process-list
   process id=process155d1d498 taskpriority=0 logused=0 waitresource=OBJECT: 26:1765581328:0  waittime=16222 ownerId=5528849 transactionname=user_transaction lasttranstarted=2019-02-07T13:39:38.837 XDES=0x15ec1c3a8 lockMode=IX schedulerid=4 kpid=8140 status=suspended spid=54 sbid=0 ecid=0 priority=0 trancount=2 lastbatchstarted=2019-02-07T13:39:38.833 lastbatchcompleted=2019-02-07T13:39:38.833 lastattention=2019-02-07T13:38:49.187 clientapp=Microsoft SQL Server Management Studio - Query hostname=pppp hostpid=12276 loginname=FINCONSGROUP\anna.savchenko isolationlevel=serializable (4) xactid=5528849 currentdb=26 lockTimeout=4294967295 clientoption1=671098976 clientoption2=390200
    executionStack
     frame procname=adhoc line=9 stmtstart=248 stmtend=334 sqlhandle=0x02000000f445021276fec0f5ec119082f65611ce316a4d280000000000000000000000000000000000000000
UPDATE t1 SET InUse=0
WHERE id IN (1,3)     
    inputbuf
SET TRAN ISOLATION LEVEL SERIALIZABLE
BEGIN TRAN
SELECT * FROM dbo.t1
WHERE id IN (1,3)
WAITFOR DELAY '00:00:20'
UPDATE t1 SET InUse=0
WHERE id IN (1,3)
COMMIT
   process id=process15649f868 taskpriority=0 logused=0 waitresource=OBJECT: 26:1765581328:0  waittime=1212 ownerId=5529084 transactionname=user_transaction lasttranstarted=2019-02-07T13:39:53.847 XDES=0x15ec1d048 lockMode=IX schedulerid=3 kpid=15516 status=suspended spid=57 sbid=0 ecid=0 priority=0 trancount=2 lastbatchstarted=2019-02-07T13:39:53.847 lastbatchcompleted=2019-02-07T13:39:53.847 lastattention=1900-01-01T00:00:00.847 clientapp=Microsoft SQL Server Management Studio - Query hostname=pppp hostpid=12276 loginname=FINCONSGROUP\anna.savchenko isolationlevel=serializable (4) xactid=5529084 currentdb=26 lockTimeout=4294967295 clientoption1=671098976 clientoption2=390200
    executionStack
     frame procname=adhoc line=9 stmtstart=248 stmtend=334 sqlhandle=0x0200000093b3ba1c08d586d1f142f473a7c8996074a369fc0000000000000000000000000000000000000000
UPDATE t1 SET InUse=0
WHERE id IN (1,3)     
    inputbuf
SET TRAN ISOLATION LEVEL SERIALIZABLE
BEGIN TRAN
SELECT * FROM dbo.t1
WHERE id IN (1,3)
WAITFOR DELAY '00:00:20'
UPDATE t1 SET InUse=0
WHERE id IN (1,3)
COMMIT    
  resource-list
   objectlock lockPartition=0 objid=1765581328 subresource=FULL dbid=26 objectname=parts.dbo.t1 id=lock152c2d300 mode=S associatedObjectId=1765581328
    owner-list
     owner id=process15649f868 mode=S
     owner id=process15649f868 mode=IX requestType=convert
    waiter-list
     waiter id=process155d1d498 mode=IX requestType=convert
   objectlock lockPartition=0 objid=1765581328 subresource=FULL dbid=26 objectname=parts.dbo.t1 id=lock152c2d300 mode=S associatedObjectId=1765581328
    owner-list
     owner id=process155d1d498 mode=S
     owner id=process155d1d498 mode=IX requestType=convert
    waiter-list
     waiter id=process15649f868 mode=IX requestType=convert