linux-kernel - Re: [syzbot] [ext4?] possible deadlock in ext4

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Message-ID: <20250825104155.5420-1-hdanton@sina.com>
Date: Mon, 25 Aug 2025 18:41:52 +0800
From: Hillf Danton <hdanton@...a.com>
To: syzbot <syzbot+c5c9c223a721d7353490@...kaller.appspotmail.com>
Cc: linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [ext4?] possible deadlock in ext4_truncate (2)

> Date: Mon, 25 Aug 2025 01:51:30 -0700	[thread overview]
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    8d245acc1e88 Merge tag 'char-misc-6.17-rc3' of git://git.k..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1654ac42580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=da02162f945f3311
> dashboard link: https://syzkaller.appspot.com/bug?extid=c5c9c223a721d7353490
> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15d1cef0580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10f4e7bc580000

#syz test

--- x/fs/ext4/inode.c
+++ y/fs/ext4/inode.c
@@ -4573,6 +4573,7 @@ int ext4_truncate(struct inode *inode)
 	struct ext4_inode_info *ei = EXT4_I(inode);
 	unsigned int credits;
 	int err = 0, err2;
+	static int subclass = 0;
 	handle_t *handle;
 	struct address_space *mapping = inode->i_mapping;
 
@@ -4636,7 +4637,7 @@ int ext4_truncate(struct inode *inode)
 	ext4_fc_track_inode(handle, inode);
 	ext4_check_map_extents_env(inode);
 
-	down_write(&EXT4_I(inode)->i_data_sem);
+	down_write_nested(&EXT4_I(inode)->i_data_sem, subclass++);
 	ext4_discard_preallocations(inode);
 
 	if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
--