Re: [PATCH] LockAcquireExtended improvement

On Tue, 28 Nov 2023 at 18:23, Jingxian Li <aqktjcm(at)qq(dot)com> wrote:
>
> Hi hackers,
>
> I found a problem when doing the test shown below:
>
> Time
>
> Session A
>
> Session B
>
> T1
>
> postgres=# create table test(a int);
>
> CREATE TABLE
>
> postgres=# insert into test values (1);
>
> INSERT 0 1
>
>
>
> T2
>
> postgres=# begin;
>
> BEGIN
>
> postgres=*# lock table test in access exclusive mode ;
>
> LOCK TABLE
>
>
>
> T3
>
>
>
> postgres=# begin;
>
> BEGIN
>
> postgres=*# lock table test in exclusive mode ;
>
> T4
>
> Case 1:
>
> postgres=*# lock table test in share row exclusive mode nowait;
>
> ERROR: could not obtain lock on relation "test"
>
> --------------------------------------------
>
> Case 2:
>
> postgres=*# lock table test in share row exclusive mode;
>
> LOCK TABLE
>
>
>
> At T4 moment in session A, (case 1) when executing SQL “lock table test in share row exclusive mode nowait;”, an error occurs with message “could not obtain lock on relation test";However, (case 2) when executing the SQL above without nowait, lock can be obtained successfully.
>
> Digging into the source code, I find that in case 2 the lock was obtained in the function ProcSleep instead of LockAcquireExtended. Due to nowait logic processed before WaitOnLock->ProcSleep, acquiring lock failed in case 1. Can any changes be made so that the act of such lock granted occurs before WaitOnLock?
>
>
>
> Providing a more universal case:
>
> Transaction A already holds an n-mode lock on table test. If then transaction A requests an m-mode lock on table Test, m and n have the following constraints:
>
> (lockMethodTable->conflictTab[n] & lockMethodTable->conflictTab[m]) == lockMethodTable->conflictTab[m]
>
> Obviously, in this case, m<=n.
>
> Should the m-mode lock be granted before WaitOnLock?
>
>
>
> In the case of m=n (i.e. we already hold the lock), the m-mode lock is immediately granted in the LocalLock path, without the need of lock conflict check.
>
> Based on the facts above, can we obtain a weaker lock (m<n) on the same object within the same transaction without doing lock conflict check?
>
> Since m=n works, m<n should certainly work too.
>
>
>
> I am attaching a patch here with which the problem in case 1 fixed.

I did not see any test added for this, should we add a test case for this?

Regards,
Vignesh