I finally found this. https://www.postgresql.org/message-id/29104.1182785028%40sss.pgh.pa.us
This is very useful to understand the Soft Block. On Wed, Jun 19, 2019 at 7:18 PM Rui Hai Jiang <ruih...@gmail.com> wrote: > Hello, hackers. > > Any body know how to produce a Soft Block case of Deadlock Detection? > I have produced the Hard Block case, but can't produce the Soft Block case. > > > I read the design: src/backend/storage/lmgr/README. It reads, > > "If a process A is behind a process B in some lock's wait queue, and > their requested locks conflict, then we must say that A waits for B, since > ProcLockWakeup will never awaken A before B. This creates additional > edges in the WFG. We call these "soft" edges, as opposed to the "hard" > edges induced by locks already held. Note that if B already holds any > locks conflicting with A's request, then their relationship is a hard edge > not a soft edge." > > > But after trying many testing, I couldn't figure out how to produce a Soft > Block. > > Following is what I did. > > * Hard Block Case > > ** Prepare > > create table t1 ( id int primary key, test int ); > create table t2 ( id int primary key, test int ); > > insert into t1 values (10,10); > insert into t2 values (20,20); > > ** test > > step1/backend1: > begin; > update t1 set test=11 where id=10; > > step2/backend2: > begin; > update t2 set test=21 where id=20; > > step3/backend1: > update t2 set test=21 where id=20; > > step4/process2: /*deadlock detected*/ > update t1 set test=11 where id=10; > > > > > * Soft Block Case > > ** Prepare > > create table t1 ( id int primary key, test int ); > > create table t2 ( id int primary key, test int ); > > create table t3 ( id int primary key, test int ); > > insert into t1 values (10,10); > insert into t2 values (20,20); > insert into t3 values (30,30); > > ** test > > step1/backend1: /*lock t1.row1*/ > begin; > select * from t1 where id=10 for update; > > > step2/backend2: /*lock t2.row1*/ > begin; > select * from t2 where id=20 for no key update; > > step3/backend3: /*lock t2.row1*/ > begin; > select * from t2 where id=20 for key share; > > step4/backend4:/*lock t3.row1*/ > begin; > select * from t3 where id=30 for update; > > step5/backend4：/*try to lock t1.row1*/ > update t1 set id=11 where id=10; > > step6/backend3：/*try to lock t3.row1*/ > update t3 set id=31 where id=30; > > step7/backend5：/*try to lock t2.row1, hope to create a soft edge*/ > begin; > update t2 set id=21 where id=20; > > step8/backend1：/*try to lock t2.row1*/ /*Expect to detect soft block, > but there seems no soft block*/ > update t2 set test=21 where id=20; > >
