ext4: publish jinode after initialization

ext4_inode_attach_jinode() publishes ei->jinode to concurrent users.
It used to set ei->jinode before jbd2_journal_init_jbd_inode(),
allowing a reader to observe a non-NULL jinode with i_vfs_inode
still unset.

The fast commit flush path can then pass this jinode to
jbd2_wait_inode_data(), which dereferences i_vfs_inode->i_mapping and
may crash.

Below is the crash I observe:
```
BUG: unable to handle page fault for address: 000000010beb47f4
PGD 110e51067 P4D 110e51067 PUD 0
Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 1 UID: 0 PID: 4850 Comm: fc_fsync_bench_ Not tainted 6.18.0-00764-g795a690c06a5 #1 PREEMPT(voluntary)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.17.0-2-2 04/01/2014
RIP: 0010:xas_find_marked+0x3d/0x2e0
Code: e0 03 48 83 f8 02 0f 84 f0 01 00 00 48 8b 47 08 48 89 c3 48 39 c6 0f 82 fd 01 00 00 48 85 c9 74 3d 48 83 f9 03 77 63 4c 8b 0f <49> 8b 71 08 48 c7 47 18 00 00 00 00 48 89 f1 83 e1 03 48 83 f9 02
RSP: 0018:ffffbbee806e7bf0 EFLAGS: 00010246
RAX: 000000000010beb4 RBX: 000000000010beb4 RCX: 0000000000000003
RDX: 0000000000000001 RSI: 0000002000300000 RDI: ffffbbee806e7c10
RBP: 0000000000000001 R08: 0000002000300000 R09: 000000010beb47ec
R10: ffff9ea494590090 R11: 0000000000000000 R12: 0000002000300000
R13: ffffbbee806e7c90 R14: ffff9ea494513788 R15: ffffbbee806e7c88
FS: 00007fc2f9e3e6c0(0000) GS:ffff9ea6b1444000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000010beb47f4 CR3: 0000000119ac5000 CR4: 0000000000750ef0
PKRU: 55555554
Call Trace:
<TASK>
filemap_get_folios_tag+0x87/0x2a0
__filemap_fdatawait_range+0x5f/0xd0
? srso_alias_return_thunk+0x5/0xfbef5
? __schedule+0x3e7/0x10c0
? srso_alias_return_thunk+0x5/0xfbef5
? srso_alias_return_thunk+0x5/0xfbef5
? srso_alias_return_thunk+0x5/0xfbef5
? preempt_count_sub+0x5f/0x80
? srso_alias_return_thunk+0x5/0xfbef5
? cap_safe_nice+0x37/0x70
? srso_alias_return_thunk+0x5/0xfbef5
? preempt_count_sub+0x5f/0x80
? srso_alias_return_thunk+0x5/0xfbef5
filemap_fdatawait_range_keep_errors+0x12/0x40
ext4_fc_commit+0x697/0x8b0
? ext4_file_write_iter+0x64b/0x950
? srso_alias_return_thunk+0x5/0xfbef5
? preempt_count_sub+0x5f/0x80
? srso_alias_return_thunk+0x5/0xfbef5
? vfs_write+0x356/0x480
? srso_alias_return_thunk+0x5/0xfbef5
? preempt_count_sub+0x5f/0x80
ext4_sync_file+0xf7/0x370
do_fsync+0x3b/0x80
? syscall_trace_enter+0x108/0x1d0
__x64_sys_fdatasync+0x16/0x20
do_syscall_64+0x62/0x2c0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
...
```

Fix this by initializing the jbd2_inode first.
Use smp_wmb() and WRITE_ONCE() to publish ei->jinode after
initialization. Readers use READ_ONCE() to fetch the pointer.

Fixes: a361293 ("jbd2: Fix oops in jbd2_journal_file_inode()")
Cc: stable@vger.kernel.org
Signed-off-by: Li Chen <me@linux.beauty>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260225082617.147957-1-me@linux.beauty
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

Author: FirstLoveLife

fs/ext4/fast_commit.c

@@ -975,13 +975,13 @@ static int ext4_fc_flush_data(journal_t *journal)
 	int ret = 0;
 
 	list_for_each_entry(ei, &sbi->s_fc_q[FC_Q_MAIN], i_fc_list) {
-		ret = jbd2_submit_inode_data(journal, ei->jinode);
+		ret = jbd2_submit_inode_data(journal, READ_ONCE(ei->jinode));
 		if (ret)
 			return ret;
 	}
 
 	list_for_each_entry(ei, &sbi->s_fc_q[FC_Q_MAIN], i_fc_list) {
-		ret = jbd2_wait_inode_data(journal, ei->jinode);
+		ret = jbd2_wait_inode_data(journal, READ_ONCE(ei->jinode));
 		if (ret)
 			return ret;
 	}

fs/ext4/inode.c

@@ -128,17 +128,19 @@ void ext4_inode_csum_set(struct inode *inode, struct ext4_inode *raw,
 static inline int ext4_begin_ordered_truncate(struct inode *inode,
 					      loff_t new_size)
 {
+	struct jbd2_inode *jinode = READ_ONCE(EXT4_I(inode)->jinode);
+
 	trace_ext4_begin_ordered_truncate(inode, new_size);
 	/*
 	 * If jinode is zero, then we never opened the file for
 	 * writing, so there's no need to call
 	 * jbd2_journal_begin_ordered_truncate() since there's no
 	 * outstanding writes we need to flush.
 	 */
-	if (!EXT4_I(inode)->jinode)
+	if (!jinode)
 		return 0;
 	return jbd2_journal_begin_ordered_truncate(EXT4_JOURNAL(inode),
-						   EXT4_I(inode)->jinode,
+						   jinode,
 						   new_size);
 }
 
@@ -4451,8 +4453,13 @@ int ext4_inode_attach_jinode(struct inode *inode)
 			spin_unlock(&inode->i_lock);
 			return -ENOMEM;
 		}
-		ei->jinode = jinode;
-		jbd2_journal_init_jbd_inode(ei->jinode, inode);
+		jbd2_journal_init_jbd_inode(jinode, inode);
+		/*
+		 * Publish ->jinode only after it is fully initialized so that
+		 * readers never observe a partially initialized jbd2_inode.
+		 */
+		smp_wmb();
+		WRITE_ONCE(ei->jinode, jinode);
 		jinode = NULL;
 	}
 	spin_unlock(&inode->i_lock);