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
