writeback: fix race that cause writeback hung
There is a race between mark inode dirty and writeback thread, see the following scenario. In this case, writeback thread will not run though there is dirty_io. __mark_inode_dirty() bdi_writeback_workfn() ... ... spin_lock(&inode->i_lock); ... if (bdi_cap_writeback_dirty(bdi)) { <<< assume wb has dirty_io, so wakeup_bdi is false. <<< the following inode_dirty also have wakeup_bdi false. if (!wb_has_dirty_io(&bdi->wb)) wakeup_bdi = true; } spin_unlock(&inode->i_lock); <<< assume last dirty_io is removed here. pages_written = wb_do_writeback(wb); ... <<< work_list empty and wb has no dirty_io, <<< delayed_work will not be queued. if (!list_empty(&bdi->work_list) || (wb_has_dirty_io(wb) && dirty_writeback_interval)) queue_delayed_work(bdi_wq, &wb->dwork, msecs_to_jiffies(dirty_writeback_interval * 10)); spin_lock(&bdi->wb.list_lock); inode->dirtied_when = jiffies; <<< new dirty_io is added. list_move(&inode->i_wb_list, &bdi->wb.b_dirty); spin_unlock(&bdi->wb.list_lock); <<< though there is dirty_io, but wakeup_bdi is false, <<< so writeback thread will not be waked up and <<< the new dirty_io will not be flushed. if (wakeup_bdi) bdi_wakeup_thread_delayed(bdi); Writeback will run until there is a new flush work queued. This may cause a lot of dirty pages stay in memory for a long time. Signed-off-by: NJunxiao Bi <junxiao.bi@oracle.com> Reviewed-by: NJan Kara <jack@suse.cz> Cc: Fengguang Wu <fengguang.wu@intel.com> Signed-off-by: NAndrew Morton <akpm@linux-foundation.org> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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