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Fix race condition between hot standby and restoring a full-page image. 

There was a window in RestoreBackupBlock where a page would be zeroed out,
but not yet locked. If a backend pinned and locked the page in that window,
it saw the zeroed page instead of the old page or new page contents, which
could lead to missing rows in a result set, or errors.

To fix, replace RBM_ZERO with RBM_ZERO_AND_LOCK, which atomically pins,
zeroes, and locks the page, if it's not in the buffer cache already.

In stable branches, the old RBM_ZERO constant is renamed to RBM_DO_NOT_USE,
to avoid breaking any 3rd party extensions that might use RBM_ZERO. More
importantly, this avoids renumbering the other enum values, which would
cause even bigger confusion in extensions that use ReadBufferExtended, but
haven't been recompiled.

Backpatch to all supported versions; this has been racy since hot standby
was introduced.
This commit is contained in:
Heikki Linnakangas 2014-11-13 19:47:44 +02:00
parent 955b4ba7f6
commit 8fc23a9ed0
6 changed files with 66 additions and 25 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
src/backend/access/hash/hashpage.c
View File

@ -155,9 +155,8 @@ _hash_getinitbuf(Relation rel, BlockNumber blkno)
if (blkno == P_NEW)
elog(ERROR, "hash AM does not use P_NEW");
buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_ZERO, NULL);
LockBuffer(buf, HASH_WRITE);
buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_ZERO_AND_LOCK,
NULL);
/* ref count and lock type are correct */
@ -198,11 +197,13 @@ _hash_getnewbuf(Relation rel, BlockNumber blkno, ForkNumber forkNum)
if (BufferGetBlockNumber(buf) != blkno)
elog(ERROR, "unexpected hash relation size: %u, should be %u",
BufferGetBlockNumber(buf), blkno);
LockBuffer(buf, HASH_WRITE);
}
else
buf = ReadBufferExtended(rel, forkNum, blkno, RBM_ZERO, NULL);
LockBuffer(buf, HASH_WRITE);
{
buf = ReadBufferExtended(rel, forkNum, blkno, RBM_ZERO_AND_LOCK,
NULL);
}
/* ref count and lock type are correct */

3
src/backend/access/heap/heapam.c
View File

@ -7527,9 +7527,8 @@ heap_xlog_newpage(XLogRecPtr lsn, XLogRecord *record)
* not do anything that assumes we are touching a heap.
*/
buffer = XLogReadBufferExtended(xlrec->node, xlrec->forknum, xlrec->blkno,
RBM_ZERO);
RBM_ZERO_AND_LOCK);
Assert(BufferIsValid(buffer));
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
page = (Page) BufferGetPage(buffer);
if (xlrec->hole_length == 0)

6
src/backend/access/transam/xlog.c
View File

@ -4054,12 +4054,8 @@ RestoreBackupBlockContents(XLogRecPtr lsn, BkpBlock bkpb, char *blk,
Page page;
buffer = XLogReadBufferExtended(bkpb.node, bkpb.fork, bkpb.block,
RBM_ZERO);
get_cleanup_lock ? RBM_ZERO_AND_CLEANUP_LOCK : RBM_ZERO_AND_LOCK);
Assert(BufferIsValid(buffer));
if (get_cleanup_lock)
LockBufferForCleanup(buffer);
else
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
page = (Page) BufferGetPage(buffer);

17
src/backend/access/transam/xlogutils.c
View File

@ -257,7 +257,8 @@ XLogCheckInvalidPages(void)
* The returned buffer is exclusively-locked.
*
* For historical reasons, instead of a ReadBufferMode argument, this only
* supports RBM_ZERO (init == true) and RBM_NORMAL (init == false) modes.
* supports RBM_ZERO_AND_LOCK (init == true) and RBM_NORMAL (init == false)
* modes.
*/
Buffer
XLogReadBuffer(RelFileNode rnode, BlockNumber blkno, bool init)
@ -265,8 +266,8 @@ XLogReadBuffer(RelFileNode rnode, BlockNumber blkno, bool init)
Buffer buf;
buf = XLogReadBufferExtended(rnode, MAIN_FORKNUM, blkno,
init ? RBM_ZERO : RBM_NORMAL);
if (BufferIsValid(buf))
init ? RBM_ZERO_AND_LOCK : RBM_NORMAL);
if (BufferIsValid(buf) && !init)
LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
return buf;
@ -285,8 +286,8 @@ XLogReadBuffer(RelFileNode rnode, BlockNumber blkno, bool init)
* dropped or truncated. If we don't see evidence of that later in the WAL
* sequence, we'll complain at the end of WAL replay.)
*
* In RBM_ZERO and RBM_ZERO_ON_ERROR modes, if the page doesn't exist, the
* relation is extended with all-zeroes pages up to the given block number.
* In RBM_ZERO_* modes, if the page doesn't exist, the relation is extended
* with all-zeroes pages up to the given block number.
*
* In RBM_NORMAL_NO_LOG mode, we return InvalidBuffer if the page doesn't
* exist, and we don't check for all-zeroes. Thus, no log entry is made
@ -340,7 +341,11 @@ XLogReadBufferExtended(RelFileNode rnode, ForkNumber forknum,
do
{
if (buffer != InvalidBuffer)
{
if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
}
buffer = ReadBufferWithoutRelcache(rnode, forknum,
P_NEW, mode, NULL);
}
@ -348,6 +353,8 @@ XLogReadBufferExtended(RelFileNode rnode, ForkNumber forknum,
/* Handle the corner case that P_NEW returns non-consecutive pages */
if (BufferGetBlockNumber(buffer) != blkno)
{
if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
buffer = ReadBufferWithoutRelcache(rnode, forknum, blkno,
mode, NULL);

41
src/backend/storage/buffer/bufmgr.c
View File

@ -213,14 +213,19 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
* valid, the page is zeroed instead of throwing an error. This is intended
* for non-critical data, where the caller is prepared to repair errors.
*
* In RBM_ZERO mode, if the page isn't in buffer cache already, it's filled
* with zeros instead of reading it from disk. Useful when the caller is
* going to fill the page from scratch, since this saves I/O and avoids
* In RBM_ZERO_AND_LOCK mode, if the page isn't in buffer cache already, it's
* filled with zeros instead of reading it from disk. Useful when the caller
* is going to fill the page from scratch, since this saves I/O and avoids
* unnecessary failure if the page-on-disk has corrupt page headers.
* The page is returned locked to ensure that the caller has a chance to
* initialize the page before it's made visible to others.
* Caution: do not use this mode to read a page that is beyond the relation's
* current physical EOF; that is likely to cause problems in md.c when
* the page is modified and written out. P_NEW is OK, though.
*
* RBM_ZERO_AND_CLEANUP_LOCK is the same as RBM_ZERO_AND_LOCK, but acquires
* a cleanup-strength lock on the page.
*
* RBM_NORMAL_NO_LOG mode is treated the same as RBM_NORMAL here.
*
* If strategy is not NULL, a nondefault buffer access strategy is used.
@ -362,6 +367,18 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
isExtend,
found);
/*
* In RBM_ZERO_AND_LOCK mode, the caller expects the buffer to
* be already locked on return.
*/
if (!isLocalBuf)
{
if (mode == RBM_ZERO_AND_LOCK)
LWLockAcquire(bufHdr->content_lock, LW_EXCLUSIVE);
else if (mode == RBM_ZERO_AND_CLEANUP_LOCK)
LockBufferForCleanup(BufferDescriptorGetBuffer(bufHdr));
}
return BufferDescriptorGetBuffer(bufHdr);
}
@ -443,8 +460,11 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
* Read in the page, unless the caller intends to overwrite it and
* just wants us to allocate a buffer.
*/
if (mode == RBM_ZERO)
if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK ||
mode == RBM_DO_NOT_USE)
{
MemSet((char *) bufBlock, 0, BLCKSZ);
}
else
{
instr_time io_start,
@ -485,6 +505,19 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
}
}
/*
* In RBM_ZERO_AND_LOCK mode, grab the buffer content lock before marking
* the page as valid, to make sure that no other backend sees the zeroed
* page before the caller has had a chance to initialize it.
*
* Since no-one else can be looking at the page contents yet, there is no
* difference between an exclusive lock and a cleanup-strength lock.
* (Note that we cannot use LockBuffer() of LockBufferForCleanup() here,
* because they assert that the buffer is already valid.)
*/
if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
LWLockAcquire(bufHdr->content_lock, LW_EXCLUSIVE);
if (isLocalBuf)
{
/* Only need to adjust flags */

11
src/include/storage/bufmgr.h
View File

@ -36,11 +36,16 @@ typedef enum BufferAccessStrategyType
typedef enum
{
RBM_NORMAL, /* Normal read */
RBM_ZERO, /* Don't read from disk, caller will
* initialize */
RBM_DO_NOT_USE, /* This used to be RBM_ZERO. Only kept for
* binary compatibility with 3rd party
* extensions. */
RBM_ZERO_ON_ERROR, /* Read, but return an all-zeros page on error */
RBM_NORMAL_NO_LOG /* Don't log page as invalid during WAL
RBM_NORMAL_NO_LOG, /* Don't log page as invalid during WAL
* replay; otherwise same as RBM_NORMAL */
RBM_ZERO_AND_LOCK, /* Don't read from disk, caller will
* initialize. Also locks the page. */
RBM_ZERO_AND_CLEANUP_LOCK /* Like RBM_ZERO_AND_LOCK, but locks the page
* in "cleanup" mode */
} ReadBufferMode;
/* in globals.c ... this duplicates miscadmin.h */