xfs_bounce_unaligned_dio_write(
	struct xfs_inode	*ip,
	xfs_fileoff_t		offset_fsb,
	struct xfs_bmbt_irec	*imap)
{
	struct xfs_bmbt_irec	irec;
	xfs_fileoff_t		delta;
	bool			shared;
	bool			x;
	int			error;

	irec = *imap;
	if (offset_fsb > irec.br_startoff) {
		delta = offset_fsb - irec.br_startoff;
		irec.br_blockcount -= delta;
		irec.br_startblock += delta;
		irec.br_startoff = offset_fsb;
	}
	error = xfs_reflink_trim_around_shared(ip, &irec, &shared, &x);
	if (error)
		return error;

	/*
	 * We're here because we're trying to do a directio write to a
	 * region that isn't aligned to a filesystem block.  If any part
	 * of the extent is shared, fall back to buffered mode to handle
	 * the RMW.  This is done by returning -EREMCHG ("remote addr
	 * changed"), which is caught further up the call stack.
	 */
	if (shared) {
		trace_xfs_reflink_bounce_dio_write(ip, imap);
		return -EREMCHG;
	}
	return 0;
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.