When checkpoint age goes beyond the sync flush threshold and
buf_flush_sync_lsn is set, page cleaner enters into "furious flush"
stage to aggressively flush dirty pages from flush list and pull
checkpoint LSN above safe margin. In this stage, page cleaner skips
doing LRU flush and eviction.

In 10.6, all other threads entirely rely on page cleaner to generate
free pages. If free pages get over while page cleaner is busy in
"furious flush" stage, a session thread could wait for free page in the
middle of a min-transaction(mtr) while holding latches on other pages.

It, in turn, can prevent page cleaner to flush such pages preventing
checkpoint LSN to move forward creating a deadlock situation. Even
otherwise, it could create a stall and hang like situation for large BP
with plenty of dirty pages to flush before the stage could finish.

Fix: During furious flush, check and evict LRU pages after each flush
iteration.