Re: Shared buffer access rule violations?

On Tue, Jul 10, 2018 at 8:33 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Asim R P <apraveen(at)pivotal(dot)io> writes:
>
>> One can find several PageInit() calls with no content lock held. See,
>> for example:
>
>> fill_seq_with_data()
>
> That would be for a relation that no one else can even see yet, no?

Yes, when the sequence is being created. No, when the sequence is
being reset, in ResetSequence().

>
>> vm_readbuf()
>> fsm_readbuf()
>
> In these cases I'd imagine that the I/O completion interlock is what
> is preventing other backends from accessing the buffer.
>

What is I/O completion interlock? I see no difference in initializing
a visimap/fsm page and initializing a standard heap page. For
standard heap pages, the code currently acquires the buffer pin as
well as content lock for initialization.

>> Moreover, fsm_vacuum_page() performs
>> "PageGetContents(page))->fp_next_slot = 0;" without content lock.
>
> That field is just a hint, IIRC, and the possibility of a torn read
> is explicitly not worried about.

Yes, that's a hint. And ignoring torn page possibility doesn't result
in checksum failures because fsm_read() passes RMB_ZERO_ON_ERROR to
buffer manager. The page will be zeroed out in the event of checksum
failure.

Asim