linux-kernel - Re: [PATCH 1/2] ocfs2: Fix deadlock in ocfs2_read_virt

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Message-ID: <2690bc1f-049f-4a27-823e-5da0ae1bdcda@suse.com>
Date: Thu, 19 Sep 2024 16:13:30 +0800
From: "heming.zhao@...e.com" <heming.zhao@...e.com>
To: Mohammed Anees <pvmohammedanees2003@...il.com>,
 ocfs2-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc: Mark Fasheh <mark@...heh.com>, Joel Becker <jlbec@...lplan.org>,
 Joseph Qi <joseph.qi@...ux.alibaba.com>,
 syzbot+18a87160c7d64ba2e2f6@...kaller.appspotmail.com
Subject: Re: [PATCH 1/2] ocfs2: Fix deadlock in ocfs2_read_virt_blocks



On 9/19/24 01:20, Mohammed Anees wrote:
> syzbot has found a kernel BUG in ocfs2_write_cluster_by_desc,
> while the next patch in the series resolves this, another
> bug has been detected due to a potential deadlock [1].
> 
> The scenario is depicted here,
> 
> 	CPU0					CPU1
> lock(&ocfs2_file_ip_alloc_sem_key);
>                                 lock(&osb->system_file_mutex);
>                                 lock(&ocfs2_file_ip_alloc_sem_key);
> lock(&osb->system_file_mutex);
> 
> The function calls which could lead to this are:
> 
> CPU0
> ocfs2_write_begin - lock(&ocfs2_file_ip_alloc_sem_key);
> .
> .
> .
> ocfs2_get_system_file_inode - lock(&osb->system_file_mutex);
> 
> CPU1 -
> ocfs2_get_system_file_inode - lock(&osb->system_file_mutex);
> .
> .
> .
> ocfs2_read_virt_blocks - lock(&ocfs2_file_ip_alloc_sem_key);
> 
> This issue can be resolved by making the down_read -> down_read_try
> in the ocfs2_read_virt_blocks.
> 
> [1] https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6

I haven't checked this patch, but in my view, following URL is correct.
https://syzkaller.appspot.com/bug?extid=e0055ea09f1f5e6fabdd

Heming
> 
> Reported-and-tested-by: syzbot+18a87160c7d64ba2e2f6@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6
> Signed-off-by: Mohammed Anees <pvmohammedanees2003@...il.com>
> ---
>   fs/ocfs2/extent_map.c | 16 +++++++++++++++-
>   1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ocfs2/extent_map.c b/fs/ocfs2/extent_map.c
> index 70a768b62..f83d0a3b6 100644
> --- a/fs/ocfs2/extent_map.c
> +++ b/fs/ocfs2/extent_map.c
> @@ -12,6 +12,7 @@
>   #include <linux/slab.h>
>   #include <linux/types.h>
>   #include <linux/fiemap.h>
> +#include <linux/delay.h>
>   
>   #include <cluster/masklog.h>
>   
> @@ -961,6 +962,8 @@ int ocfs2_read_virt_blocks(struct inode *inode, u64 v_block, int nr,
>   	int rc = 0;
>   	u64 p_block, p_count;
>   	int i, count, done = 0;
> +	int retries, max_retries = 5;
> +	int retry_delay_ms = 30;
>   
>   	trace_ocfs2_read_virt_blocks(
>   	     inode, (unsigned long long)v_block, nr, bhs, flags,
> @@ -973,7 +976,18 @@ int ocfs2_read_virt_blocks(struct inode *inode, u64 v_block, int nr,
>   	}
>   
>   	while (done < nr) {
> -		down_read(&OCFS2_I(inode)->ip_alloc_sem);
> +		retries = 0;
> +		while (retries < max_retries) {
> +			if (down_read_trylock(&OCFS2_I(inode)->ip_alloc_sem))
> +				break; // Lock acquired
> +			msleep(retry_delay_ms);
> +			retries++;
> +		}
> +		if (retries == max_retries) {
> +			rc = -EAGAIN;
> +			mlog(ML_ERROR, "Cannot acquire lock\n");
> +			break;
> +		}
>   		rc = ocfs2_extent_map_get_blocks(inode, v_block + done,
>   						 &p_block, &p_count, NULL);
>   		up_read(&OCFS2_I(inode)->ip_alloc_sem);