Re: [PATCH V2 1/6] null_blk: use DEFINE_MUTEX for the file-scope mutex

[Damien Le Moal <dlemoal@kernel.org>]

On 7/7/26 11:55, Zizhi Wo wrote:
> From: Zizhi Wo <wozizhi@huawei.com>
> 
> In null_init(), mutex_init(&lock) currently happens after
> configfs_register_subsystem(), which exposes the nullb subsystem to
> userspace. A racing mkdir() into /sys/kernel/config/nullb/ can reach
> null_find_dev_by_name() -> mutex_lock(&lock) before the mutex is
> initialized, trigger warning:
> 
> [  123.137788] DEBUG_LOCKS_WARN_ON(lock->magic != lock)
> [  123.137796] WARNING: kernel/locking/mutex.c:159 at mutex_lock+0x171/0x1c0, CPU#13: mkdir/1301
> [  123.140090] Modules linked in: null_blk(+) nft_fib_inet nft_fib_ipv4
> ......
> [  123.154926] Call Trace:
> [  123.155172]  <TASK>
> [  123.155419]  ? __pfx_mutex_lock+0x10/0x10
> [  123.156181]  ? __pfx__raw_spin_lock+0x10/0x10
> [  123.156571]  nullb_group_make_group+0x20/0x100 [null_blk]
> [  123.157011]  configfs_mkdir+0x47b/0xc70
> [  123.157337]  ? __pfx_configfs_mkdir+0x10/0x10
> [  123.157719]  ? may_create_dentry+0x242/0x2e0
> [  123.158061]  vfs_mkdir+0x2a9/0x6c0
> [  123.158352]  filename_mkdirat+0x3dc/0x500
> [  123.158710]  ? __pfx_filename_mkdirat+0x10/0x10
> [  123.159070]  ? strncpy_from_user+0x3a/0x1d0
> [  123.159413]  __x64_sys_mkdir+0x6b/0x90
> [  123.159760]  do_syscall_64+0xea/0x600
> 
> Replace the runtime mutex_init(&lock) with a static DEFINE_MUTEX(lock)
> declaration to fix this issue.
> 
> Fixes: 49c3b9266a71 ("block: null_blk: Improve device creation with configfs")
> Suggested-by: Bart Van Assche <bvanassche@acm.org>
> Signed-off-by: Zizhi Wo <wozizhi@huawei.com>
> Reviewed-by: Bart Van Assche <bvanassche@acm.org>

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>

-- 
Damien Le Moal
Western Digital Research