Re: [PATCH] Identify LWLocks in tracepoints

On Mon, 22 Mar 2021 at 17:00, Peter Eisentraut <
peter(dot)eisentraut(at)enterprisedb(dot)com> wrote:

> On 20.03.21 01:29, Craig Ringer wrote:
> > There is already support for that. See the documentation at the end
> of
> > this page:
> >
> https://www.postgresql.org/docs/devel/dynamic-trace.html#DEFINING-TRACE-POINTS
> > <
> https://www.postgresql.org/docs/devel/dynamic-trace.html#DEFINING-TRACE-POINTS
> >
> >
> >
> > Pretty sure it won't work right now.
> >
> > To use systemtap semaphores (the _ENABLED macros) you need to run dtrace
> > -g to generate a probes.o then link that into postgres.
> >
> > I don't think we do that. I'll double check soon.
>
> We do that. (It's -G.)
>

Huh. I could've sworn we didn't. My mistake, it's there in
src/backend/Makefile .

In that case I'll amend the patch to use semaphore guards.

(On a side note, systemtap's semaphore support is actually a massive pain.
The way it's implemented in <sys/sdt.h> means that a single compilation
unit may not use both probes.d style markers produced by the dtrace script
and use regular DTRACE_PROBE(providername,probename) preprocessor macros.
If it attempts to do so, the DTRACE_PROBE macros will emit inline asm that
tries to reference probename_semaphore symbols that will not exist,
resulting in linker errors or runtime link errors. But that's really a
systemtap problem. Core PostgreSQL doesn't use any explicit
DTRACE_PROBE(...), STAP_PROBE(...) etc.)