Re: Expose lock group leader pid in pg_stat_activity

On Tue, Jan 28, 2020 at 12:36:41PM +0100, Julien Rouhaud wrote:
> So, AFAICT the LockHashPartitionLockByProc is required when
> iterating/modifying lockGroupMembers or lockGroupLink, but just
> getting the leader pid should be safe.

This still seems unsafe:

git show -U11 -w --patience b025f32e0b src/backend/utils/adt/pgstatfuncs.c
+ /*
+ * If a PGPROC entry was retrieved, display wait events and lock
+ * group leader information if any. To avoid extra overhead, no
+ * extra lock is being held, so there is no guarantee of
+ * consistency across multiple rows.
+ */
...
+ PGPROC *leader;
...
+ leader = proc->lockGroupLeader;
+ if (leader)
+ {
# does something guarantee that leader doesn't change ?
+ values[29] = Int32GetDatum(leader->pid);
+ nulls[29] = false;
}

Michael seems to have raised the issue:

On Thu, Jan 16, 2020 at 04:49:12PM +0900, Michael Paquier wrote:
> And actually, the way you are looking at the leader's PID is visibly
> incorrect and inconsistent because the patch takes no shared LWLock on
> the leader using LockHashPartitionLockByProc() followed by
> LWLockAcquire(), no? That's incorrect because it could be perfectly
> possible to crash with this code between the moment you check if
> lockGroupLeader is NULL and the moment you look at
> lockGroupLeader->pid if a process is being stopped in-between and
> removes itself from a lock group in ProcKill().

But I don't see how it was addressed ?

I read this:

src/backend/storage/lmgr/lock.c: * completely valid. We cannot safely dereference another backend's
src/backend/storage/lmgr/lock.c- * lockGroupLeader field without holding all lock partition locks, and
src/backend/storage/lmgr/lock.c- * it's not worth that.)

I think if you do:
|LWLockAcquire(&proc->backendLock, LW_SHARED);
..then it's at least *safe* to access leader->pid, but it may be inconsistent
unless you also call LockHashPartitionLockByProc.

I wasn't able to produce a crash, so maybe I missed something.

--
Justin