Postgresql源码（13）Pin/UnpinBuffer流程图

static bool
PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy)
{
	Buffer		b = BufferDescriptorGetBuffer(buf);
	bool		result;
	PrivateRefCountEntry *ref;

	ref = GetPrivateRefCountEntry(b, true);

	if (ref == NULL)
	{
		uint32		buf_state;
		uint32		old_buf_state;

		ReservePrivateRefCountEntry();
		ref = NewPrivateRefCountEntry(b);

		old_buf_state = pg_atomic_read_u32(&buf->state);
		for (;;)
		{
			if (old_buf_state & BM_LOCKED)
				old_buf_state = WaitBufHdrUnlocked(buf);

			buf_state = old_buf_state;

			/* increase refcount */
			buf_state += BUF_REFCOUNT_ONE;

			if (strategy == NULL)
			{
				/* Default case: increase usagecount unless already max. */
				if (BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT)
					buf_state += BUF_USAGECOUNT_ONE;
			}
			else
			{
				/*
				 * Ring buffers shouldn't evict others from pool.  Thus we
				 * don't make usagecount more than 1.
				 */
				if (BUF_STATE_GET_USAGECOUNT(buf_state) == 0)
					buf_state += BUF_USAGECOUNT_ONE;
			}

			if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
											   buf_state))
			{
				result = (buf_state & BM_VALID) != 0;
				break;
			}
		}
	}
	else
	{
		/* If we previously pinned the buffer, it must surely be valid */
		result = true;
	}

	ref->refcount++;
	Assert(ref->refcount > 0);
	ResourceOwnerRememberBuffer(CurrentResourceOwner, b);
	return result;
}

static void
UnpinBuffer(BufferDesc *buf, bool fixOwner)
{
	PrivateRefCountEntry *ref;
	Buffer		b = BufferDescriptorGetBuffer(buf);

	/* not moving as we're likely deleting it soon anyway */
	ref = GetPrivateRefCountEntry(b, false);
	Assert(ref != NULL);

	if (fixOwner)
		ResourceOwnerForgetBuffer(CurrentResourceOwner, b);

	Assert(ref->refcount > 0);
	ref->refcount--;
	if (ref->refcount == 0)
	{
		uint32		buf_state;
		uint32		old_buf_state;

		/* I'd better not still hold any locks on the buffer */
		Assert(!LWLockHeldByMe(BufferDescriptorGetContentLock(buf)));
		Assert(!LWLockHeldByMe(BufferDescriptorGetIOLock(buf)));

		/*
		 * Decrement the shared reference count.
		 *
		 * Since buffer spinlock holder can update status using just write,
		 * it's not safe to use atomic decrement here; thus use a CAS loop.
		 */
		old_buf_state = pg_atomic_read_u32(&buf->state);
		for (;;)
		{
			if (old_buf_state & BM_LOCKED)
				old_buf_state = WaitBufHdrUnlocked(buf);

			buf_state = old_buf_state;

			buf_state -= BUF_REFCOUNT_ONE;

			if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
											   buf_state))
				break;
		}

		/* Support LockBufferForCleanup() */
		if (buf_state & BM_PIN_COUNT_WAITER)
		{
			/*
			 * Acquire the buffer header lock, re-check that there's a waiter.
			 * Another backend could have unpinned this buffer, and already
			 * woken up the waiter.  There's no danger of the buffer being
			 * replaced after we unpinned it above, as it's pinned by the
			 * waiter.
			 */
			buf_state = LockBufHdr(buf);

			if ((buf_state & BM_PIN_COUNT_WAITER) &&
				BUF_STATE_GET_REFCOUNT(buf_state) == 1)
			{
				/* we just released the last pin other than the waiter's */
				int			wait_backend_pid = buf->wait_backend_pid;

				buf_state &= ~BM_PIN_COUNT_WAITER;
				UnlockBufHdr(buf, buf_state);
				ProcSendSignal(wait_backend_pid);
			}
			else
				UnlockBufHdr(buf, buf_state);
		}
		ForgetPrivateRefCountEntry(ref);
	}
}