Обсуждение: 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
Вложения
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