Re: Multixact slru doesn't don't force WAL flushes in SlruPhysicalWritePage() - Mailing list pgsql-hackers

On Tue, Nov 10, 2015 at 11:22:47PM -0500, Noah Misch wrote:
> On Mon, Nov 09, 2015 at 10:40:07PM +0100, Andres Freund wrote:
> >     /*
> >      * Optional array of WAL flush LSNs associated with entries in the SLRU
> >      * pages.  If not zero/NULL, we must flush WAL before writing pages (true
> >      * for pg_clog, false for multixact, pg_subtrans, pg_notify).  group_lsn[]
> >      * has lsn_groups_per_page entries per buffer slot, each containing the
> >      * highest LSN known for a contiguous group of SLRU entries on that slot's
> >      * page.
> >      */
> >     XLogRecPtr *group_lsn;
> >     int            lsn_groups_per_page;
> >

> Here's the multixact.c comment justifying it:
>
>  * XLOG interactions: this module generates an XLOG record whenever a new
>  * OFFSETs or MEMBERs page is initialized to zeroes, as well as an XLOG record
>  * whenever a new MultiXactId is defined.  This allows us to completely
>  * rebuild the data entered since the last checkpoint during XLOG replay.
>  * Because this is possible, we need not follow the normal rule of
>  * "write WAL before data"; the only correctness guarantee needed is that
>  * we flush and sync all dirty OFFSETs and MEMBERs pages to disk before a
>  * checkpoint is considered complete.  If a page does make it to disk ahead
>  * of corresponding WAL records, it will be forcibly zeroed before use anyway.
>  * Therefore, we don't need to mark our pages with LSN information; we have
>  * enough synchronization already.
>
> The comment's justification is incomplete, though.  What of pages filled over
> the course of multiple checkpoint cycles?

Having studied this further, I think it is safe for a reason given elsewhere:

     * Note: we need not flush this XLOG entry to disk before proceeding. The
     * only way for the MXID to be referenced from any data page is for
     * heap_lock_tuple() to have put it there, and heap_lock_tuple() generates
     * an XLOG record that must follow ours.  The normal LSN interlock between
     * the data page and that XLOG record will ensure that our XLOG record
     * reaches disk first.  If the SLRU members/offsets data reaches disk
     * sooner than the XLOG record, we do not care because we'll overwrite it
     * with zeroes unless the XLOG record is there too; see notes at top of
     * this file.

I find no flaw in the first three sentences.  In most cases, one of
multixact_redo(), TrimMultiXact() or ExtendMultiXactOffset() will indeed
overwrite the widowed mxid data with zeroes before the system again writes
data in that vicinity.  We can fail to do that if a backend stalls just after
calling GetNewMultiXactId(), then recovers and updates SLRU pages long after
other backends filled the balance of those pages.  That's still okay; if we
never flush the XLOG_MULTIXACT_CREATE_ID record, no xmax referring to the mxid
will survive recovery.  I drafted the attached comment update to consolidate
and slightly correct this justification.  (If we were designing the multixact
subsystem afresh today, I'd vote for following the write-WAL-before-data rule
despite believing it is not needed.  The simplicity would be worth it.)

While contemplating the "stalls ... just after calling GetNewMultiXactId()"
scenario, I noticed a race condition not involving any write-WAL-before-data
violation.  MultiXactIdCreateFromMembers() and callees have this skeleton:

GetNewMultiXactId
  LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE)
  ExtendMultiXactOffset()
  ExtendMultiXactMember()
  START_CRIT_SECTION()
  (MultiXactState->nextMXact)++
  MultiXactState->nextOffset += nmembers
  LWLockRelease(MultiXactGenLock)
XLogInsert(RM_MULTIXACT_ID, XLOG_MULTIXACT_CREATE_ID)
RecordNewMultiXact
  LWLockAcquire(MultiXactOffsetControlLock, LW_EXCLUSIVE)
  *offptr = offset;  /* update pg_multixact/offsets SLRU page */
  LWLockRelease(MultiXactOffsetControlLock)
  LWLockAcquire(MultiXactMemberControlLock, LW_EXCLUSIVE)
  *memberptr = members[i].xid;  /* update pg_multixact/members SLRU page */
  *flagsptr = flagsval;  /* update pg_multixact/members SLRU page */
  LWLockRelease(MultiXactMemberControlLock)
END_CRIT_SECTION

Between GetNewMultiXactId() and XLogInsert(), other backends will be free to
create higher-numbered mxids.  They will skip over SLRU space the current
backend reserved.  Future GetMultiXactIdMembers() calls rely critically on the
current backend eventually finishing:

     * 2. The next multixact may still be in process of being filled in: that
     * is, another process may have done GetNewMultiXactId but not yet written
     * the offset entry for that ID.  In that scenario, it is guaranteed that
     * the offset entry for that multixact exists (because GetNewMultiXactId
     * won't release MultiXactGenLock until it does) but contains zero
     * (because we are careful to pre-zero offset pages). Because
     * GetNewMultiXactId will never return zero as the starting offset for a
     * multixact, when we read zero as the next multixact's offset, we know we
     * have this case.  We sleep for a bit and try again.

A crash while paused this way makes permanent the zero offset.  After
recovery, though no xmax carries the offset-zero mxid, GetMultiXactIdMembers()
on the immediate previous mxid hangs indefinitely.  Also,
pg_get_multixact_members(zero_offset_mxid) gets an assertion failure.  I have
not thoroughly considered how best to fix these.  Test procedure:

-- backend 1
checkpoint;
create table victim0 (c) as select 1;
create table stall1 (c) as select 1;
create table last2 (c) as select 1;
begin;
select c from victim0 for key share;
select c from stall1 for key share;
select c from last2 for key share;

-- backend 2
begin; update victim0 set c = c + 1; rollback; -- burn one mxid
-- In a shell, attach GDB to backend 2.  GDB will stop the next SQL command at
-- XLogInsert() in MultiXactIdCreateFromMembers():
--   gdb --pid $BACKEND_PID -ex 'b XLogInsert' -ex c
select c from stall1 for key share; -- stall in gdb while making an mxid

-- backend 3
select c from last2 for key share; -- use one more mxid; flush WAL

-- in GDB session, issue command: kill

-- backend 1, after recovery
select c from victim0;        -- hangs, uncancelable

-- backend 2, after recovery: assertion failure
select pg_get_multixact_members((xmax::text::bigint - 1)::text::xid) from last2;