Add API to check if an existing exclusive lock allows cleanup.

LockBufferForCleanup() acquires a cleanup lock unconditionally, and
ConditionalLockBufferForCleanup() acquires a cleanup lock if it is
possible to do so without waiting; this patch adds a new API,
IsBufferCleanupOK(), which tests whether an exclusive lock already
held happens to be a cleanup lock.  This is possible because a cleanup
lock simply means an exclusive lock plus the assurance any other pins
on the buffer are newer than our own pin.  Therefore, just as the
existing functions decide that the exclusive lock that they've just
taken is a cleanup lock if they observe the pin count to be 1, this
new function allows us to observe that the pin count is 1 on a buffer
we've already locked.

This is useful in situations where a backend definitely wishes to
modify the buffer and also wishes to perform cleanup operations if
possible.  The patch to eliminate heavyweight locking by hash indexes
uses this, and it may have other applications as well.

Amit Kapila, per a suggestion from me.  Some comment adjustments by me
as well.

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index df4c9d71094b35a9b38ae66003198ebd15a47d3f..58b0a975c0f08bd31640ecc68e48238294dae305 100644 (file)
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3745,6 +3745,55 @@ ConditionalLockBufferForCleanup(Buffer buffer)
    return false;
 }
 
+/*
+ * IsBufferCleanupOK - as above, but we already have the lock
+ *
+ * Check whether it's OK to perform cleanup on a buffer we've already
+ * locked.  If we observe that the pin count is 1, our exclusive lock
+ * happens to be a cleanup lock, and we can proceed with anything that
+ * would have been allowable had we sought a cleanup lock originally.
+ */
+bool
+IsBufferCleanupOK(Buffer buffer)
+{
+   BufferDesc *bufHdr;
+   uint32      buf_state;
+
+   Assert(BufferIsValid(buffer));
+
+   if (BufferIsLocal(buffer))
+   {
+       /* There should be exactly one pin */
+       if (LocalRefCount[-buffer - 1] != 1)
+           return false;
+       /* Nobody else to wait for */
+       return true;
+   }
+
+   /* There should be exactly one local pin */
+   if (GetPrivateRefCount(buffer) != 1)
+       return false;
+
+   bufHdr = GetBufferDescriptor(buffer - 1);
+
+   /* caller must hold exclusive lock on buffer */
+   Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
+                               LW_EXCLUSIVE));
+
+   buf_state = LockBufHdr(bufHdr);
+
+   Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+   if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+   {
+       /* pincount is OK. */
+       UnlockBufHdr(bufHdr, buf_state);
+       return true;
+   }
+
+   UnlockBufHdr(bufHdr, buf_state);
+   return false;
+}
+
 
 /*
  * Functions for buffer I/O handling

diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 7b6ba960001ec986c7f35290c3ea6cee55d60db2..821bee5eceaf5f26bd0be2766a8ecbd213449041 100644 (file)
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -227,6 +227,7 @@ extern void LockBuffer(Buffer buffer, int mode);
 extern bool ConditionalLockBuffer(Buffer buffer);
 extern void LockBufferForCleanup(Buffer buffer);
 extern bool ConditionalLockBufferForCleanup(Buffer buffer);
+extern bool IsBufferCleanupOK(Buffer buffer);
 extern bool HoldingBufferPinThatDelaysRecovery(void);
 
 extern void AbortBufferIO(void);