Re: "ERROR: latch already owned" on gharial

On 08/02/2024 04:08, Soumyadeep Chakraborty wrote:
> A possible ordering of events:
>
> (1) DisownLatch() is called by pid Y during ProcKill() and the write for
> latch->owner_pid = 0 is NOT yet flushed to shmem.
>
> (2) The PGPROC object for pid Y is returned to the free list.
>
> (3) Pid X sees the same PGPROC object on the free list and grabs it.
>
> (4) Pid X does sanity check inside OwnLatch during InitProcess and
> still sees the
> old value of latch->owner_pid = Y (and not = 0), and trips the ERROR.
>
> The above sequence of operations should apply to PG HEAD as well.
>
> Suggestion:
>
> Should we do a pg_memory_barrier() at the end of DisownLatch(), like in
> ResetLatch(), like the one introduced in [3]? This would ensure that the write
> latch->owner_pid = 0; is flushed to shmem. The attached patch does this.

Hmm, there is a pair of SpinLockAcquire() and SpinLockRelease() in
ProcKill(), before step 3 can happen. Comment in spin.h about
SpinLockAcquire/Release:

> * Load and store operations in calling code are guaranteed not to be
> * reordered with respect to these operations, because they include a
> * compiler barrier. (Before PostgreSQL 9.5, callers needed to use a
> * volatile qualifier to access data protected by spinlocks.)

That talks about a compiler barrier, though, not a memory barrier. But
looking at the implementations in s_lock.h, I believe they do act as
memory barrier, too.

So you might indeed have that problem on 9.4, but AFAICS not on later
versions.

--
Heikki Linnakangas
Neon (https://neon.tech)