From 8fc23a9ed0a040d039431ef79b1bf166395ed180 Mon Sep 17 00:00:00 2001 From: Heikki Linnakangas Date: Thu, 13 Nov 2014 19:47:44 +0200 Subject: [PATCH] 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. --- src/backend/access/hash/hashpage.c | 13 ++++---- src/backend/access/heap/heapam.c | 3 +- src/backend/access/transam/xlog.c | 6 +--- src/backend/access/transam/xlogutils.c | 17 +++++++---- src/backend/storage/buffer/bufmgr.c | 41 +++++++++++++++++++++++--- src/include/storage/bufmgr.h | 11 +++++-- 6 files changed, 66 insertions(+), 25 deletions(-) diff --git a/src/backend/access/hash/hashpage.c b/src/backend/access/hash/hashpage.c index 9e4a2e0434..0aaa15c389 100644 --- a/src/backend/access/hash/hashpage.c +++ b/src/backend/access/hash/hashpage.c @@ -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 */ diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c index fa608cf8fd..62c790b2a2 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -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) diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c index e689b4b9f0..7f6d20a542 100644 --- a/src/backend/access/transam/xlog.c +++ b/src/backend/access/transam/xlog.c @@ -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); diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c index b7829ff4c6..9c8dce9532 100644 --- a/src/backend/access/transam/xlogutils.c +++ b/src/backend/access/transam/xlogutils.c @@ -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); diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index f5da54472c..74ef1cbc62 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -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 */ diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h index 89447d0b3d..921e4edde2 100644 --- a/src/include/storage/bufmgr.h +++ b/src/include/storage/bufmgr.h @@ -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 */