linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: remove circular locking between sb_internal and fs

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Date:   Thu, 21 Oct 2021 20:11:20 +0800
From:   Chao Yu <chao@...nel.org>
To:     Daeho Jeong <daeho43@...il.com>, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com
Cc:     Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [f2fs-dev] [PATCH] f2fs: remove circular locking between
 sb_internal and fs_reclaim

On 2021/10/15 3:05, Daeho Jeong wrote:
> From: Daeho Jeong <daehojeong@...gle.com>
> 
> We detected the below circular locking dependency between sb_internal
> and fs_reclaim. So, removed it by calling dquot_initialize() before
> sb_start_intwrite().
> 
>   ======================================================
>   WARNING: possible circular locking dependency detected
>   ------------------------------------------------------
>   kswapd0/133 is trying to acquire lock:
> ffffff80d5fb9680 (sb_internal#2){.+.+}-{0:0}, at: evict+0xd4/0x2f8
> 
> but task is already holding lock:
> ffffffda597c93a8 (fs_reclaim){+.+.}-{0:0}, at:
> __fs_reclaim_acquire+0x4/0x50
> 
> which lock already depends on the new lock.
> ...
> other info that might help us debug this:
> 
>   Chain exists of:
> 
> sb_internal#2 --> &s->s_dquot.dqio_sem --> fs_reclaim
> 
>    Possible unsafe locking scenario:
> 
>          CPU0                    CPU1
>          ----                    ----
>     lock(fs_reclaim);
>                                  lock(&s->s_dquot.dqio_sem);
>                                  lock(fs_reclaim);
>     lock(sb_internal#2);

Sorry, I still didn't get the root cause of this deadlock issue, could
you please explain more about this?

And why calling dquot_initialize() in drop_inode() could break the
circular locking dependency?

Thanks,

> 
> Signed-off-by: Daeho Jeong <daehojeong@...gle.com>
> ---
>   fs/f2fs/super.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index 86eeb019cc52..a133932333c5 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -1370,6 +1370,8 @@ static int f2fs_drop_inode(struct inode *inode)
>   			/* should remain fi->extent_tree for writepage */
>   			f2fs_destroy_extent_node(inode);
>   
> +			dquot_initialize(inode);
> +
>   			sb_start_intwrite(inode->i_sb);
>   			f2fs_i_size_write(inode, 0);
>   
>