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

Hallvard Breien Furuseth wrote:
On 02/08/14 18:57, Howard Chu wrote:
Hallvard Breien Furuseth wrote:
If so, MDB_NOLOCK may be in trouble since it uses pick_meta()
instead of mti_txnid.  Should there be a separate CACHEFLUSH after
writing the datapages if MDB_NOLOCK, and the current CACHEFLUSH
should just flush the metapages?

I don't see any reason for that. As always, the only thing that matters
is when the metapages get written.

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.

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. 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.)

Delaying the mti_txnid change
protects from that, except when something does not use mti_txnid -
hence the concern for MDB_NOLOCK using mdb_env_pick_meta().

Hmm.  There are some other places that use pick_meta even when
the lockfile is in use.  Maybe they too should try (mti_txnid & 1).

Does the code contradict this comment above, or is it about
something else?
/* Memory ordering issues are irrelevant ... */

Quite simply, on MIPS, write()s into the buffer cache aren't coherent
with the on-chip data (or instruction, but irrelevant) cache.

  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/