Re: [syzbot ci] Re: blk-mq: fix possible deadlocks

[Ming Lei <ming.lei@redhat.com>]

On Mon, Dec 01, 2025 at 03:50:22AM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2025/11/30 18:09, syzbot ci 写道:
> > syzbot ci has tested the following series
> >
> > [v3] blk-mq: fix possible deadlocks
> > https://lore.kernel.org/all/20251130024349.2302128-1-yukuai@fnnas.com
> > * [PATCH v3 01/10] blk-mq-debugfs: factor out a helper to register debugfs for all rq_qos
> > * [PATCH v3 02/10] blk-rq-qos: fix possible debugfs_mutex deadlock
> > * [PATCH v3 03/10] blk-mq-debugfs: make blk_mq_debugfs_register_rqos() static
> > * [PATCH v3 04/10] blk-mq-debugfs: warn about possible deadlock
> > * [PATCH v3 05/10] block/blk-rq-qos: add a new helper rq_qos_add_frozen()
> > * [PATCH v3 06/10] blk-wbt: fix incorrect lock order for rq_qos_mutex and freeze queue
> > * [PATCH v3 07/10] blk-iocost: fix incorrect lock order for rq_qos_mutex and freeze queue
> > * [PATCH v3 08/10] blk-iolatency: fix incorrect lock order for rq_qos_mutex and freeze queue
> > * [PATCH v3 09/10] blk-throttle: remove useless queue frozen
> > * [PATCH v3 10/10] block/blk-rq-qos: cleanup rq_qos_add()
> >
> > and found the following issue:
> > possible deadlock in pcpu_alloc_noprof
> >
> > Full report is available here:
> > https://ci.syzbot.org/series/1aec77f0-c53f-4b3b-93fb-b3853983b6bd
> >
> > ***
> >
> > possible deadlock in pcpu_alloc_noprof
> >
> > tree:      linux-next
> > URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
> > base:      7d31f578f3230f3b7b33b0930b08f9afd8429817
> > arch:      amd64
> > compiler:  Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> > config:    https://ci.syzbot.org/builds/70dca9e4-6667-4930-9024-150d656e503e/config
> >
> > soft_limit_in_bytes is deprecated and will be removed. Please report your usecase to linux-mm@kvack.org if you depend on this functionality.
> > ======================================================
> > WARNING: possible circular locking dependency detected
> > syzkaller #0 Not tainted
> > ------------------------------------------------------
> > syz-executor/6047 is trying to acquire lock:
> > ffffffff8e04f760 (fs_reclaim){+.+.}-{0:0}, at: prepare_alloc_pages+0x152/0x650
> >
> > but task is already holding lock:
> > ffffffff8e02dde8 (pcpu_alloc_mutex){+.+.}-{4:4}, at: pcpu_alloc_noprof+0x25b/0x1750
> >
> > which lock already depends on the new lock.
> >
> >
> > the existing dependency chain (in reverse order) is:
> >
> > -> #2 (pcpu_alloc_mutex){+.+.}-{4:4}:
> >         __mutex_lock+0x187/0x1350
> >         pcpu_alloc_noprof+0x25b/0x1750
> >         blk_stat_alloc_callback+0xd5/0x220
> >         wbt_init+0xa3/0x500
> >         wbt_enable_default+0x25d/0x350
> >         blk_register_queue+0x36a/0x3f0
> >         __add_disk+0x677/0xd50
> >         add_disk_fwnode+0xfc/0x480
> >         loop_add+0x7f0/0xad0
> >         loop_init+0xd9/0x170
> >         do_one_initcall+0x1fb/0x820
> >         do_initcall_level+0x104/0x190
> >         do_initcalls+0x59/0xa0
> >         kernel_init_freeable+0x334/0x4b0
> >         kernel_init+0x1d/0x1d0
> >         ret_from_fork+0x599/0xb30
> >         ret_from_fork_asm+0x1a/0x30
> >
> > -> #1 (&q->q_usage_counter(io)#17){++++}-{0:0}:
> >         blk_alloc_queue+0x538/0x620
> >         __blk_mq_alloc_disk+0x15c/0x340
> >         loop_add+0x411/0xad0
> >         loop_init+0xd9/0x170
> >         do_one_initcall+0x1fb/0x820
> >         do_initcall_level+0x104/0x190
> >         do_initcalls+0x59/0xa0
> >         kernel_init_freeable+0x334/0x4b0
> >         kernel_init+0x1d/0x1d0
> >         ret_from_fork+0x599/0xb30
> >         ret_from_fork_asm+0x1a/0x30
> >
> > -> #0 (fs_reclaim){+.+.}-{0:0}:
> >         __lock_acquire+0x15a6/0x2cf0
> >         lock_acquire+0x117/0x340
> >         fs_reclaim_acquire+0x72/0x100
> >         prepare_alloc_pages+0x152/0x650
> >         __alloc_frozen_pages_noprof+0x123/0x370
> >         __alloc_pages_noprof+0xa/0x30
> >         pcpu_populate_chunk+0x182/0xb30
> >         pcpu_alloc_noprof+0xcb6/0x1750
> >         xt_percpu_counter_alloc+0x161/0x220
> >         translate_table+0x1323/0x2040
> >         ip6t_register_table+0x106/0x7d0
> >         ip6table_nat_table_init+0x43/0x2e0
> >         xt_find_table_lock+0x30c/0x3e0
> >         xt_request_find_table_lock+0x26/0x100
> >         do_ip6t_get_ctl+0x730/0x1180
> >         nf_getsockopt+0x26e/0x290
> >         ipv6_getsockopt+0x1ed/0x290
> >         do_sock_getsockopt+0x2b4/0x3d0
> >         __x64_sys_getsockopt+0x1a5/0x250
> >         do_syscall_64+0xfa/0xf80
> >         entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > other info that might help us debug this:
> >
> > Chain exists of:
> >    fs_reclaim --> &q->q_usage_counter(io)#17 --> pcpu_alloc_mutex
> >
> >   Possible unsafe locking scenario:
> >
> >         CPU0                    CPU1
> >         ----                    ----
> >    lock(pcpu_alloc_mutex);
> >                                 lock(&q->q_usage_counter(io)#17);
> >                                 lock(pcpu_alloc_mutex);
> >    lock(fs_reclaim);
> 
> This does not look like introduced by this set, wbt_init() will hold
> pcpu_alloc_mutex, and it can be called with queue frozen without this
> set.

It is introduced by your patch 6 in which blk_mq_freeze_queue() is added
before calling wb_init() from wbt_enable_default(), then the warning is
triggered.



Thanks,
Ming