Re: [sqlsmith] Unpinning error in parallel worker

On Tue, Apr 24, 2018, at 16:06, Thomas Munro wrote:
> On Wed, Apr 25, 2018 at 2:21 AM, Jonathan Rudenberg
> <jonathan(at)titanous(dot)com> wrote:
> > This issue happened again in production, here are the stack traces from three we grabbed before nuking the >400 hanging backends.
> >
> > [...]
> > #4 0x000055fccb93b21c in LWLockAcquire+188() at /usr/lib/postgresql/10/bin/postgres at lwlock.c:1233
> > #5 0x000055fccb925fa7 in dsm_create+151() at /usr/lib/postgresql/10/bin/postgres at dsm.c:493
> > #6 0x000055fccb6f2a6f in InitializeParallelDSM+511() at /usr/lib/postgresql/10/bin/postgres at parallel.c:266
> > [...]
>
> Thank you. These stacks are all blocked trying to acquire
> DynamicSharedMemoryControlLock. My theory is that they can't because
> one backend -- the one that emitted the error "FATAL: cannot unpin a
> segment that is not pinned" -- is deadlocked against itself. After
> emitting that error you can see from Andreas's "seabisquit" stack that
> that shmem_exit() runs dsm_backend_shutdown() which runs dsm_detach()
> which tries to acquire DynamicSharedMemoryControlLock again, even
> though we already hold it at that point.
>
> I'll write a patch to fix that unpleasant symptom. While holding
> DynamicSharedMemoryControlLock we shouldn't raise any errors without
> releasing it first, because the error handling path will try to
> acquire it again. That's a horrible failure mode as you have
> discovered.
>
> But that isn't the root problem: we shouldn't be raising that error,
> and I'd love to see the stack of the one process that did that and
> then self-deadlocked. I will have another go at trying to reproduce
> it here today.

Thanks for the update!

We have turned off parallel queries (using max_parallel_workers_per_gather = 0) for now, as the production impact of this bug is unfortunately quite problematic.

What will this failure look like with the patch you've proposed?

Thanks again,

Jonathan