Re: openldap.git branch mdb.master updated. 3630066843b7ca6b2cd12911d3e2fe3314cd4549

[Hallvard Breien Furuseth <h.b.furuseth@usit.uio.no>]

On 02/08/14 22:31, Howard Chu wrote:
> Hallvard Breien Furuseth wrote:
> > What matters is that nothing *sees* the metapage before its data
> > pages, nor sees the mti_txnid change before the metapage.  I thought
> > that's what cache coherency and memory ordering was about.
>
> No, memory ordering is about seeing partial changes to a single page,
> outside of the order that the program wrote it. Which can happen on
> superscalar chips with out-of-order execution.

OK, that explains half of that code, thanks:-)

> > So to explain my previous message a bit: A cacheflush() which flushes
> > a metapage and its datapages all in one chunk makes me nervous.  If
> > that's necessary (rather than just flushing the meta at that point), I
> > imagine that just before the flush, it's possible for something to see
> > the metapage before its datapages.
>
> Not possible. The cacheflush is atomic.

Fine, but it's the situation before the flush which worries
me, when the cache can be incoherent.

> Note that this is invalidating
> an on-chip data cache which is typically only 32KB or so. It has nothing
> to do with flushing the buffer cache. ("flush" is a misnomer, but that's
> what the syscall is called.)

OK, but apparently it's still a cache which can include
data from both metapages and datapages.

Or is this only relevant for the same thread, and not other
threads/processes (read-only txns starting before the flush)?
In that case it would be safe.