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

[Chao Yu via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

On 2026/3/4 12:57, Shinichiro Kawasaki wrote:
> 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.

Shinichiro,

IMO, this looks like a false alarm of lockdep, what do you think of this fix?

https://git.kernel.org/pub/scm/linux/kernel/git/chao/linux.git/commit/?h=bugfix/syzbot&id=3b19564b95e9ba9803ef30e90eace0977b9d140d

---
  fs/f2fs/f2fs.h  | 3 +++
  fs/f2fs/super.c | 3 +++
  2 files changed, 6 insertions(+)

diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index bb34e864d0ef..5b400e99f332 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -2042,6 +2042,9 @@ struct f2fs_sb_info {
         spinlock_t iostat_lat_lock;
         struct iostat_lat_info *iostat_io_lat;
  #endif
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+       struct lock_class_key cp_global_sem_key;
+#endif
  };

  /* Definitions to access f2fs_sb_info */
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 8774c60b4be4..9e85f31fa828 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -4948,6 +4948,9 @@ static int f2fs_fill_super(struct super_block *sb, struct fs_context *fc)
         init_f2fs_rwsem_trace(&sbi->gc_lock, sbi, LOCK_NAME_GC_LOCK);
         mutex_init(&sbi->writepages);
         init_f2fs_rwsem_trace(&sbi->cp_global_sem, sbi, LOCK_NAME_CP_GLOBAL);
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+       lockdep_set_class(&sbi->cp_global_sem, &sbi->cp_global_sem_key);
+#endif
         init_f2fs_rwsem_trace(&sbi->node_write, sbi, LOCK_NAME_NODE_WRITE);
         init_f2fs_rwsem_trace(&sbi->node_change, sbi, LOCK_NAME_NODE_CHANGE);
         spin_lock_init(&sbi->stat_lock);
-- 
2.49.0




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