Acquire read lock on a buffer while writing it out, to prevent
concurrent modifications to the page by other backends.
This commit is contained in:
Tom Lane
parent
384d94efdd
commit
a8405cfc4d
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@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $Header: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v 1.82 2000/06/08 22:37:20 momjian Exp $
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* $Header: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v 1.83 2000/09/25 04:11:09 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -429,17 +429,10 @@ BufferAlloc(Relation reln,
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inProgress = FALSE;
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for (buf = (BufferDesc *) NULL; buf == (BufferDesc *) NULL;)
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{
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/* GetFreeBuffer will abort if it can't find a free buffer */
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buf = GetFreeBuffer();
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/*
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* But it can return buf == NULL if we are in aborting transaction
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* now and so elog(ERROR,...) in GetFreeBuffer will not abort
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* again.
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*/
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if (buf == NULL)
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return NULL;
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/* GetFreeBuffer will abort if it can't find a free buffer */
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Assert(buf);
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/*
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* There should be exactly one pin on the buffer after it is
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@ -789,11 +782,21 @@ FlushBuffer(Buffer buffer, bool release)
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WaitIO(bufHdr, BufMgrLock); /* confirm end of IO */
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bufHdr->flags &= ~BM_JUST_DIRTIED;
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StartBufferIO(bufHdr, false); /* output IO start */
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SpinRelease(BufMgrLock);
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/*
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* Grab a read lock on the buffer to ensure that no
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* other backend changes its contents while we write it;
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* see comments in BufferSync().
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*/
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LockBuffer(BufferDescriptorGetBuffer(bufHdr), BUFFER_LOCK_SHARE);
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status = smgrflush(DEFAULT_SMGR, bufrel, bufHdr->tag.blockNum,
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(char *) MAKE_PTR(bufHdr->data));
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LockBuffer(BufferDescriptorGetBuffer(bufHdr), BUFFER_LOCK_UNLOCK);
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/* drop relcache refcnt incremented by RelationIdCacheGetRelation */
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RelationDecrementReferenceCount(bufrel);
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@ -1017,19 +1020,6 @@ ClearBufferDirtiedByMe(Buffer buffer, BufferDesc *bufHdr)
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* that have been dirtied by the current xact and flush them to disk.
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* We do *not* flush dirty buffers that have been dirtied by other xacts.
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* (This is a substantial change from pre-7.0 behavior.)
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*
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* OLD COMMENTS (do these still apply?)
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*
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* Also, we need to be sure that no other transaction is
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* modifying the page as we flush it. This is only a problem for objects
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* that use a non-two-phase locking protocol, like btree indices. For
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* those objects, we would like to set a write lock for the duration of
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* our IO. Another possibility is to code updates to btree pages
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* carefully, so that writing them out out of order cannot cause
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* any unrecoverable errors.
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*
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* I don't want to think hard about this right now, so I will try
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* to come back to it later.
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*/
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static void
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BufferSync()
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@ -1112,15 +1102,28 @@ BufferSync()
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bufHdr->flags &= ~BM_JUST_DIRTIED;
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StartBufferIO(bufHdr, false); /* output IO start */
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SpinRelease(BufMgrLock);
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/*
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* Grab a read lock on the buffer to ensure that no
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* other backend changes its contents while we write it;
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* otherwise we could write a non-self-consistent page
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* image to disk, which'd be bad news if the other
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* transaction aborts before writing its changes.
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*
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* Note that we still need the BM_JUST_DIRTIED mechanism
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* in case someone dirties the buffer just before we
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* grab this lock or just after we release it.
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*/
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LockBuffer(BufferDescriptorGetBuffer(bufHdr),
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BUFFER_LOCK_SHARE);
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/*
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* If we didn't have the reldesc in our local cache,
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* write this page out using the 'blind write' storage
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* manager routine. If we did find it, use the
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* standard interface.
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*/
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#ifndef OPTIMIZE_SINGLE
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SpinRelease(BufMgrLock);
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#endif /* OPTIMIZE_SINGLE */
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if (reln == (Relation) NULL)
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{
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status = smgrblindwrt(DEFAULT_SMGR,
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@ -1137,9 +1140,14 @@ BufferSync()
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bufHdr->tag.blockNum,
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(char *) MAKE_PTR(bufHdr->data));
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}
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#ifndef OPTIMIZE_SINGLE
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/*
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* Release the per-buffer readlock, reacquire BufMgrLock.
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*/
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LockBuffer(BufferDescriptorGetBuffer(bufHdr),
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BUFFER_LOCK_UNLOCK);
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SpinAcquire(BufMgrLock);
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#endif /* OPTIMIZE_SINGLE */
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UnpinBuffer(bufHdr);
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if (status == SM_FAIL)
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@ -1522,9 +1530,14 @@ BufferReplace(BufferDesc *bufHdr)
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/* To check if block content changed while flushing. - vadim 01/17/97 */
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bufHdr->flags &= ~BM_JUST_DIRTIED;
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#ifndef OPTIMIZE_SINGLE
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SpinRelease(BufMgrLock);
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#endif /* OPTIMIZE_SINGLE */
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/*
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* Grab a read lock on the buffer to ensure that no
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* other backend changes its contents while we write it;
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* see comments in BufferSync().
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*/
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LockBuffer(BufferDescriptorGetBuffer(bufHdr), BUFFER_LOCK_SHARE);
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if (reln != (Relation) NULL)
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{
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@ -1540,9 +1553,9 @@ BufferReplace(BufferDesc *bufHdr)
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false); /* no fsync */
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}
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#ifndef OPTIMIZE_SINGLE
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LockBuffer(BufferDescriptorGetBuffer(bufHdr), BUFFER_LOCK_UNLOCK);
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SpinAcquire(BufMgrLock);
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#endif /* OPTIMIZE_SINGLE */
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/* drop relcache refcnt incremented by RelationIdCacheGetRelation */
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if (reln != (Relation) NULL)
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