On Fri, Jun 24, 2016 at 11:04 AM, Amit Kapila <amit.kapil...@gmail.com> wrote:
> On Fri, Jun 24, 2016 at 4:33 AM, Andres Freund <and...@anarazel.de> wrote:
>> On 2016-06-23 18:59:57 -0400, Alvaro Herrera wrote:
>>> Andres Freund wrote:
>>>
>>> > I'm looking into three approaches right now:
>>> >
>>> > 3) Use WAL logging for the already_marked = true case.
>>>
>>>
>>> > 3) This approach so far seems the best. It's possible to reuse the
>>> > xl_heap_lock record (in an afaics backwards compatible manner), and in
>>> > most cases the overhead isn't that large. It's of course annoying to
>>> > emit more WAL, but it's not that big an overhead compared to extending a
>>> > file, or to toasting. It's also by far the simplest fix.
>>>
>
> +1 for proceeding with Approach-3.
>
>>> I suppose it's fine if we crash midway from emitting this wal record and
>>> the actual heap_update one, since the xmax will appear to come from an
>>> aborted xid, right?
>>
>> Yea, that should be fine.
>>
>>
>>> I agree that the overhead is probably negligible, considering that this
>>> only happens when toast is invoked. It's probably not as great when the
>>> new tuple goes to another page, though.
>>
>> I think it has to happen in both cases unfortunately. We could try to
>> add some optimizations (e.g. only release lock & WAL log if the target
>> page, via fsm, is before the current one), but I don't really want to go
>> there in the back branches.
>>
>
> You are right, I think we can try such an optimization in Head and
> that too if we see a performance hit with adding this new WAL in
> heap_update.
>
>
+1 for #3 approach, and attached draft patch for that.
I think attached patch would fix this problem but please let me know
if this patch is not what you're thinking.
Regards,
--
Masahiko Sawada
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 57da57a..2f3fd83 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -3923,6 +3923,28 @@ l2:
if (need_toast || newtupsize > pagefree)
{
+ /*
+ * To prevent data corruption due to updating old tuple by
+ * other backends after released buffer, we need to emit that
+ * xmax of old tuple is set and clear visibility map bits if
+ * needed before relasing buffer. We can reuse xl_heap_lock
+ * for this pupose. It should be fine even if we crash midway
+ * from this section and the actual updating one later, since
+ * the xmax will appear to come from an aborted xid.
+ */
+ START_CRIT_SECTION();
+
+ /* Celar PD_ALL_VISIBLE flags */
+ if (PageIsAllVisible(BufferGetPage(buffer)))
+ {
+ all_visible_cleared = true;
+ PageClearAllVisible(BufferGetPage(buffer));
+ visibilitymap_clear(relation, BufferGetBlockNumber(buffer),
+ vmbuffer);
+ }
+
+ MarkBufferDirty(buffer);
+
/* Clear obsolete visibility flags ... */
oldtup.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
oldtup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
@@ -3936,6 +3958,26 @@ l2:
/* temporarily make it look not-updated */
oldtup.t_data->t_ctid = oldtup.t_self;
already_marked = true;
+
+ if (RelationNeedsWAL(relation))
+ {
+ xl_heap_lock xlrec;
+ XLogRecPtr recptr;
+
+ XLogBeginInsert();
+ XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
+
+ xlrec.offnum = ItemPointerGetOffsetNumber(&oldtup.t_self);
+ xlrec.locking_xid = xid;
+ xlrec.infobits_set = compute_infobits(oldtup.t_data->t_infomask,
+ oldtup.t_data->t_infomask2);
+ XLogRegisterData((char *) &xlrec, SizeOfHeapLock);
+ recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_LOCK);
+ PageSetLSN(page, recptr);
+ }
+
+ END_CRIT_SECTION();
+
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
/*
--
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