On 5/10/22 7:38 AM, Zhihao Cheng wrote:
> Commit 505a666ee3fc ("writeback: plug writeback in wb_writeback() and
> writeback_inodes_wb()") has us holding a plug during wb_writeback, which
> may cause a potential ABBA dead lock:
>
> wb_writeback fat_file_fsync
> blk_start_plug(&plug)
> for (;;) {
> iter i-1: some reqs have been added into plug->mq_list // LOCK A
> iter i:
> progress = __writeback_inodes_wb(wb, work)
> . writeback_sb_inodes // fat's bdev
> . __writeback_single_inode
> . . generic_writepages
> . . __block_write_full_page
> . . . . __generic_file_fsync
> . . . . sync_inode_metadata
> . . . . writeback_single_inode
> . . . . __writeback_single_inode
> . . . . fat_write_inode
> . . . . __fat_write_inode
> . . . . sync_dirty_buffer // fat's bdev
> . . . . lock_buffer(bh) // LOCK B
> . . . . submit_bh
> . . . . blk_mq_get_tag // LOCK A
> . . . trylock_buffer(bh) // LOCK B
> . . . redirty_page_for_writepage
> . . . wbc->pages_skipped++
> . . --wbc->nr_to_write
> . wrote += write_chunk - wbc.nr_to_write // wrote > 0
> . requeue_inode
> . redirty_tail_locked
> if (progress) // progress > 0
> continue;
> iter i+1:
> queue_io
> // similar process with iter i, infinite for-loop !
> }
> blk_finish_plug(&plug) // flush plug won't be called
>
> Above process triggers a hungtask like:
> [ 399.044861] INFO: task bb:2607 blocked for more than 30 seconds.
> [ 399.046824] Not tainted 5.18.0-rc1-00005-gefae4d9eb6a2-dirty
> [ 399.051539] task:bb state:D stack: 0 pid: 2607 ppid:
> 2426 flags:0x00004000
> [ 399.051556] Call Trace:
> [ 399.051570] __schedule+0x480/0x1050
> [ 399.051592] schedule+0x92/0x1a0
> [ 399.051602] io_schedule+0x22/0x50
> [ 399.051613] blk_mq_get_tag+0x1d3/0x3c0
> [ 399.051640] __blk_mq_alloc_requests+0x21d/0x3f0
> [ 399.051657] blk_mq_submit_bio+0x68d/0xca0
> [ 399.051674] __submit_bio+0x1b5/0x2d0
> [ 399.051708] submit_bio_noacct+0x34e/0x720
> [ 399.051718] submit_bio+0x3b/0x150
> [ 399.051725] submit_bh_wbc+0x161/0x230
> [ 399.051734] __sync_dirty_buffer+0xd1/0x420
> [ 399.051744] sync_dirty_buffer+0x17/0x20
> [ 399.051750] __fat_write_inode+0x289/0x310
> [ 399.051766] fat_write_inode+0x2a/0xa0
> [ 399.051783] __writeback_single_inode+0x53c/0x6f0
> [ 399.051795] writeback_single_inode+0x145/0x200
> [ 399.051803] sync_inode_metadata+0x45/0x70
> [ 399.051856] __generic_file_fsync+0xa3/0x150
> [ 399.051880] fat_file_fsync+0x1d/0x80
> [ 399.051895] vfs_fsync_range+0x40/0xb0
> [ 399.051929] __x64_sys_fsync+0x18/0x30
>
> In my test, 'need_resched()' (which is imported by 590dca3a71 "fs-writeback:
> unplug before cond_resched in writeback_sb_inodes") in function
> 'writeback_sb_inodes()' seldom comes true, unless cond_resched() is deleted
> from write_cache_pages().
>
> Fix it by correcting wrote number according number of skipped pages
> in writeback_sb_inodes().
>
> Goto Link to find a reproducer.
I can take this one for 5.19, thanks.
--
Jens Axboe
