Re: [f2fs-dev] [PATCH v2 1/1] f2fs: fix lockdep WARN of sbi->cp_global_sem and q->q_usage_counter

[Shinichiro Kawasaki via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

On Feb 27, 2026 / 20:32, Jaegeuk Kim wrote:
> On 02/24, Shinichiro Kawasaki wrote:
> > On Feb 24, 2026 / 03:26, Jaegeuk Kim wrote:
> > > On 02/18, Shin'ichiro Kawasaki wrote:
> > > > From: Shin'ichiro Kawasaki via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>
> > > > 
> > > > A lockdep WARN is observed recently under the following steps:
> > > > 
> > > > 1) Create a zoned TCMU device
> > > > 2) Create a f2fs filesystem on the zoned TCMU device and mount it
> > > > 3) Fill the filesystem with files and trigger GC
> > > > 4) Unmout the filesystem
> > > > 5) Remove the zoned TCMU device
> > > > 
> > > > The lockdep WARN indicates that a circular lock depedency formed by four
> > > > contexts, as described below.
> > > > 
> > > > a) TCMU device removal context:
> > > >  - call del_gendisk() to get q->q_usage_counter
> > > >  - call start_flush_work() to get work_completion of wb->dwork
> > > > b) f2fs writeback context:
> > > >  - in wb_workfn(), which holds work_completion of wb->dwork
> > > >  - call f2fs_balance_fs() to get sbi->gc_lock
> > > > c) f2fs vfs_write context:
> > > >  - call f2fs_gc() to get sbi->gc_lock
> > > >  - call f2fs_write_checkpoint() to get sbi->cp_global_sem
> > > > d) f2fs mount context:
> > > >  - call recover_fsync_data() to get sbi->cp_global_sem
> > > >  - call f2fs_check_and_fix_write_pointer() to call blkdev_report_zones()
> > > >    that goes down to blk_mq_alloc_request and get q->q_usage_counter
> > > > 
> > > > To suppress the WARN, cut the dependency d) between sbi->cp_global_sem
> > > > and q->q_usage_counter. For that purpose, move the
> > > > f2fs_check_and_fix_write_pointer() call outside of the critical section
> > > > of sbi->cp_global_sem in f2fs_recovery_fsync_data(). This change is fine
> > > > because the write pointer fix operation only affects the main segments
> > > > and does not interact with the check point metadata. Furthermore,
> > > > conflicts between the write pointer fix operation and data/node flush
> > > > operations remain protected by SBI_POR_DOING.
> > > > 
> > > > Fixes: c426d99127b1 ("f2fs: Check write pointer consistency of open zones")
> > > > Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
> > > > Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
> > > > ---
> > > >  fs/f2fs/recovery.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/fs/f2fs/recovery.c b/fs/f2fs/recovery.c
> > > > index a26071f2b0bc..87fd6cd436fe 100644
> > > > --- a/fs/f2fs/recovery.c
> > > > +++ b/fs/f2fs/recovery.c
> > > > @@ -922,6 +922,8 @@ int f2fs_recover_fsync_data(struct f2fs_sb_info *sbi, bool check_only)
> > > >  		truncate_inode_pages_final(META_MAPPING(sbi));
> > > >  	}
> > > >  
> > > > +	f2fs_up_write_trace(&sbi->cp_global_sem, &lc);
> > > > +
> > > >  	/*
> > > >  	 * If fsync data succeeds or there is no fsync data to recover,
> > > >  	 * and the f2fs is not read only, check and fix zoned block devices'
> > > > @@ -933,8 +935,6 @@ int f2fs_recover_fsync_data(struct f2fs_sb_info *sbi, bool check_only)
> > > >  	if (!err)
> > > >  		clear_sbi_flag(sbi, SBI_POR_DOING);
> > > >  
> > > > -	f2fs_up_write_trace(&sbi->cp_global_sem, &lc);
> > > > -
> > > 
> > > This was a guard to prevent checkpoint during f2fs_check_and_fix_write_pointer()
> > > where it changes the checkpoint as well?
> > 
> > I checked f2fs_check_and_fix_write_pointer() again, and it does not look
> > changing the checkpoint to me. FYI, here I show the rough function call chain
> > from f2fs_check_and_fix_write_pointer() as below. I guess this call chain does
> > not change the checkpoint, but if I misunderstand anything, please let me know.
> > 
> >  f2fs_check_and_fix_write_pointer()
> >   fix_curseg_write_pointer()
> >    do_fix_curseg_write_pointer()
> >     blkdev_report_zones()
> >      report_one_zone_cb()
> >     f2fs_allocate_new_section()
> >      __allocate_new_segment()
> >       new_curseg()
> 
> E.g., curseg.
> 

Thanks, I looked in do_checkpoint() in fs/f2fs/checkpoing.c, and found it refers
to cursegs. Then, this patch will allow recording the cursegs in parallel of
f2fs_check_and_fix_write_pointer(), and it will results in inconsistent curseg
values in checkpoints. Not good. Let me drop this patch.

I will seek out other ways to avoid the lockdep. I have no idea how to do that
at this moment, though.

_______________________________________________
Linux-f2fs-devel mailing list
Linux-f2fs-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel