Re: Shared buffer access rule violations?

Asim R P <apraveen(at)pivotal(dot)io> writes:
> 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().

ResetSequence creates a new relfilenode, which no one else will be able
to see until it commits, so the case is effectively the same as for
creation.

>>> 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?

Oh ... the RBM_ZERO_ON_ERROR action should be done under the I/O lock,
but the ReadBuffer caller isn't holding that lock anymore, so I see your
point here. Probably, nobody's noticed because it's a corner case that
shouldn't happen under normal use, but it's not safe. I think what we
want is more like

if (PageIsNew(BufferGetPage(buf)))
{
LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
if (PageIsNew(BufferGetPage(buf)))
PageInit(BufferGetPage(buf), BLCKSZ, 0);
UnlockReleaseBuffer(buf);
}

to ensure that the page is initialized once and only once, even if
several backends do this concurrently.

regards, tom lane