Re: [PATCH] Identify LWLocks in tracepoints

On Wed, 14 Apr 2021 at 10:41, Craig Ringer
<craig(dot)ringer(at)enterprisedb(dot)com> wrote:
> On Wed, 14 Apr 2021 at 02:25, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > You could try to identify locks by pointer addresses, but that's got
> > security hazards and the addreses aren't portable across all the
> > backends involved in the parallel query because of how DSM works, so
> > it's not really that helpful in terms of matching stuff up.
>
> What I'm doing now is identifying them by LWLock* across backends. I
> keep track of DSM segment mappings in each backend inside the trace
> script and I relocate LWLock* pointers known to be inside DSM segments
> relative to a dummy base address so they're equal across backends.

BTW, one of the reasons I did this was to try to identify BDR and
pglogical code that blocks or sleeps while holding a LWLock. I got
stuck on that for other reasons, so it didn't go anywhere, but those
issues are now resolved so I should probably return to it at some
point.

It'd be a nice thing to be able to run on postgres itself too.