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Commitf2e6a2c

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Add API to check if an existing exclusive lock allows cleanup.
LockBufferForCleanup() acquires a cleanup lock unconditionally, andConditionalLockBufferForCleanup() acquires a cleanup lock if it ispossible to do so without waiting; this patch adds a new API,IsBufferCleanupOK(), which tests whether an exclusive lock alreadyheld happens to be a cleanup lock. This is possible because a cleanuplock simply means an exclusive lock plus the assurance any other pinson the buffer are newer than our own pin. Therefore, just as theexisting functions decide that the exclusive lock that they've justtaken is a cleanup lock if they observe the pin count to be 1, thisnew function allows us to observe that the pin count is 1 on a bufferwe've already locked.This is useful in situations where a backend definitely wishes tomodify the buffer and also wishes to perform cleanup operations ifpossible. The patch to eliminate heavyweight locking by hash indexesuses this, and it may have other applications as well.Amit Kapila, per a suggestion from me. Some comment adjustments by meas well.
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‎src/backend/storage/buffer/bufmgr.c

Lines changed: 49 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -3745,6 +3745,55 @@ ConditionalLockBufferForCleanup(Buffer buffer)
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return false;
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}
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/*
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* IsBufferCleanupOK - as above, but we already have the lock
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*
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* Check whether it's OK to perform cleanup on a buffer we've already
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* locked. If we observe that the pin count is 1, our exclusive lock
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* happens to be a cleanup lock, and we can proceed with anything that
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* would have been allowable had we sought a cleanup lock originally.
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*/
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bool
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IsBufferCleanupOK(Bufferbuffer)
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{
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BufferDesc*bufHdr;
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uint32buf_state;
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Assert(BufferIsValid(buffer));
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if (BufferIsLocal(buffer))
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{
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/* There should be exactly one pin */
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if (LocalRefCount[-buffer-1]!=1)
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return false;
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/* Nobody else to wait for */
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return true;
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}
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/* There should be exactly one local pin */
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if (GetPrivateRefCount(buffer)!=1)
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return false;
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bufHdr=GetBufferDescriptor(buffer-1);
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/* caller must hold exclusive lock on buffer */
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Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
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LW_EXCLUSIVE));
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buf_state=LockBufHdr(bufHdr);
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Assert(BUF_STATE_GET_REFCOUNT(buf_state)>0);
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if (BUF_STATE_GET_REFCOUNT(buf_state)==1)
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{
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/* pincount is OK. */
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UnlockBufHdr(bufHdr,buf_state);
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return true;
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}
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UnlockBufHdr(bufHdr,buf_state);
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return false;
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}
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37483797

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/*
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*Functions for buffer I/O handling

‎src/include/storage/bufmgr.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -227,6 +227,7 @@ extern void LockBuffer(Buffer buffer, int mode);
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externboolConditionalLockBuffer(Bufferbuffer);
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externvoidLockBufferForCleanup(Bufferbuffer);
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externboolConditionalLockBufferForCleanup(Bufferbuffer);
230+
externboolIsBufferCleanupOK(Bufferbuffer);
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externboolHoldingBufferPinThatDelaysRecovery(void);
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externvoidAbortBufferIO(void);

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