pgstattuple locking fix
Here is a trivial fix of locking issue in pgstattuple().
It was locking buffers around PageGetHeapFreeSpace()
in the heap scan loop, but not in the scanning of the tail.
I think we need locks in the tail, too.
Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center
Attachments:
pgstattuple_lock.patchapplication/octet-stream; name=pgstattuple_lock.patchDownload+3-3
ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.jp> writes:
Here is a trivial fix of locking issue in pgstattuple().
Hmm, is this really a bug, and if so how far back does it go?
I'm thinking that having a pin on the buffer should be enough to
call PageGetHeapFreeSpace.
regards, tom lane
Tom Lane <tgl@sss.pgh.pa.us> wrote:
ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.jp> writes:
Here is a trivial fix of locking issue in pgstattuple().
Hmm, is this really a bug, and if so how far back does it go?
I'm thinking that having a pin on the buffer should be enough to
call PageGetHeapFreeSpace.
Hmm... we might use pd_upper and pd_lower at different times,
but the error is ok for pgstattuple purpose.
(It might be dangerous for tuple insertion, though.)
Inconsistent usage of locks is confusable -- remove them, please.
Index: contrib/pgstattuple/pgstattuple.c
===================================================================
--- contrib/pgstattuple/pgstattuple.c (HEAD)
+++ contrib/pgstattuple/pgstattuple.c (working copy)
@@ -289,9 +289,7 @@
while (block <= tupblock)
{
buffer = ReadBuffer(rel, block);
- LockBuffer(buffer, BUFFER_LOCK_SHARE);
stat.free_space += PageGetHeapFreeSpace((Page) BufferGetPage(buffer));
- LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
block++;
}Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center
ITAGAKI Takahiro wrote:
Tom Lane <tgl@sss.pgh.pa.us> wrote:
ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.jp> writes:
Here is a trivial fix of locking issue in pgstattuple().
Hmm, is this really a bug, and if so how far back does it go?
I'm thinking that having a pin on the buffer should be enough to
call PageGetHeapFreeSpace.Hmm... we might use pd_upper and pd_lower at different times,
but the error is ok for pgstattuple purpose.
(It might be dangerous for tuple insertion, though.)
Inconsistent usage of locks is confusable -- remove them, please.
No I think the original patch was right. You can indeed read
inconsistent values for pd_upper and pd_lower, though the window is very
small. But a bigger issue is that in 8.3 PageGetHeapFreeSpace counts the
used line pointers to see if MaxHeapTuplesPerPage has been reached, and
I'm not convinced you can do that safely without holding a lock.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
Heikki Linnakangas wrote:
ITAGAKI Takahiro wrote:
Tom Lane <tgl@sss.pgh.pa.us> wrote:
ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.jp> writes:
Here is a trivial fix of locking issue in pgstattuple().
Hmm, is this really a bug, and if so how far back does it go?
I'm thinking that having a pin on the buffer should be enough to
call PageGetHeapFreeSpace.Hmm... we might use pd_upper and pd_lower at different times,
but the error is ok for pgstattuple purpose.
(It might be dangerous for tuple insertion, though.)
Inconsistent usage of locks is confusable -- remove them, please.No I think the original patch was right. You can indeed read
inconsistent values for pd_upper and pd_lower, though the window is very
small. But a bigger issue is that in 8.3 PageGetHeapFreeSpace counts the
used line pointers to see if MaxHeapTuplesPerPage has been reached, and
I'm not convinced you can do that safely without holding a lock.
On second thought, we do call PageGetHeapFreeSpace without holding a
lock in heap_page_prune_opt as well, so it better be safe. Looking
closer at PageGetHeapFreeSpace, I think it is. The
return value can be bogus, of course.
That's worth noting in the comments:
*** src/backend/storage/page/bufpage.c 21 Sep 2007 21:25:42 -0000 1.75
--- src/backend/storage/page/bufpage.c 22 Oct 2007 10:06:02 -0000
***************
*** 506,511 ****
--- 506,514 ----
* or dead line pointers it'd be possible to have too many line pointers.
* To avoid breaking code that assumes MaxHeapTuplesPerPage is a hard
limit
* on the number of line pointers, we make this extra check.)
+ *
+ * You don't need to hold a lock on the page to call this function,
but if
+ * you don't, the return value should be considered a hint only.
*/
Size
PageGetHeapFreeSpace(Page page)--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
"Heikki Linnakangas" <heikki@enterprisedb.com> writes:
ITAGAKI Takahiro wrote:
Tom Lane <tgl@sss.pgh.pa.us> wrote:
I'm thinking that having a pin on the buffer should be enough to
call PageGetHeapFreeSpace.Hmm... we might use pd_upper and pd_lower at different times,
No I think the original patch was right. You can indeed read
inconsistent values for pd_upper and pd_lower, though the window is very
small.
Good point. I'll apply the original patch and also Heikki's suggested
comment.
regards, tom lane
"Heikki Linnakangas" <heikki@enterprisedb.com> writes:
On second thought, we do call PageGetHeapFreeSpace without holding a
lock in heap_page_prune_opt as well, so it better be safe. Looking
closer at PageGetHeapFreeSpace, I think it is. The
return value can be bogus, of course.
That's worth noting in the comments:
On closer look, the comments in heap_page_prune_opt seem to address
the issue already, and I'm not sure why we should comment
PageGetHeapFreeSpace this way but not all the other variants in
bufpage.c.
regards, tom lane