From: syzbot <syzbot+144a1c0e22e53a08ef5a@syzkaller.appspotmail.com>
To: axboe@kernel.dk, cgroups@vger.kernel.org, josef@toxicpanda.com,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
syzkaller-bugs@googlegroups.com, tj@kernel.org
Subject: Re: [syzbot] [block?] [cgroups?] possible deadlock in blkg_conf_prep
Date: Fri, 21 Aug 2026 02:20:39 -0700 [thread overview]
Message-ID: <6a881867.dbb3a75c.13dd47.000f.GAE@google.com> (raw)
In-Reply-To: <6a807bfd.f7a79266.2f965f.0008.GAE@google.com>
syzbot has found a reproducer for the following issue on:
HEAD commit: 818bebeb63dd drm/xe: Don't hand out the flat CCS storage a..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=112c9e79580000
kernel config: https://syzkaller.appspot.com/x/.config?x=ccca94d2c01b9e78
dashboard link: https://syzkaller.appspot.com/bug?extid=144a1c0e22e53a08ef5a
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=152c9e79580000
Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-818bebeb.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6e4100526b12/vmlinux-818bebeb.xz
kernel image: https://storage.googleapis.com/syzbot-assets/723c4783ee96/bzImage-818bebeb.xz
IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+144a1c0e22e53a08ef5a@syzkaller.appspotmail.com
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.3.20/6039 is trying to acquire lock:
ffffffff8efc6c00 (fs_reclaim
){+.+.}-{0:0}, at: might_alloc include/linux/sched/mm.h:314 [inline]
){+.+.}-{0:0}, at: slab_pre_alloc_hook mm/slub.c:4539 [inline]
){+.+.}-{0:0}, at: slab_alloc_node mm/slub.c:4899 [inline]
){+.+.}-{0:0}, at: kmem_cache_alloc_noprof+0x63/0x6a0 mm/slub.c:4933
but task is already holding lock:
ffff888028bae100 (&q->blkcg_mutex){+.+.}-{4:4}, at: blkg_conf_prep+0xbc/0xf10 block/blk-cgroup.c:849
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&q->blkcg_mutex){+.+.}-{4:4}
:
lock_acquire kernel/locking/lockdep.c:5886 [inline]
lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5843
__mutex_lock_common kernel/locking/mutex.c:646 [inline]
__mutex_lock+0x1a4/0x1bc0 kernel/locking/mutex.c:821
blkcg_activate_policy+0x34f/0xbf0 block/blk-cgroup.c:1608
blk_throtl_init+0x39a/0x5b0 block/blk-throttle.c:1339
tg_set_conf.isra.0+0x252/0x500 block/blk-throttle.c:1368
cgroup_file_write+0x1e6/0x8a0 kernel/cgroup/cgroup.c:4412
kernfs_fop_write_iter+0x3e0/0x5f0 fs/kernfs/file.c:345
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x6af/0x1050 fs/read_write.c:687
ksys_write+0x12a/0x250 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
-> #1 (&q->q_usage_counter(io)#2){++++}-{0:0}:
lock_acquire kernel/locking/lockdep.c:5886 [inline]
lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5843
blk_alloc_queue+0x610/0x790 block/blk-core.c:504
__blk_alloc_disk+0xa0/0x170 block/genhd.c:1520
brd_alloc.isra.0+0x41a/0x820 drivers/block/brd.c:327
brd_init+0x167/0x190 drivers/block/brd.c:420
do_one_initcall+0x11c/0x6f0 init/main.c:1353
do_initcall_level init/main.c:1415 [inline]
do_initcalls init/main.c:1431 [inline]
do_basic_setup init/main.c:1451 [inline]
kernel_init_freeable+0x6ea/0x7b0 init/main.c:1666
kernel_init+0x21/0x1e0 init/main.c:1556
ret_from_fork+0x730/0xd60 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
-> #0 (fs_reclaim){+.+.}-{0:0}:
check_prev_add+0xeb/0xe60 kernel/locking/lockdep.c:3181
check_prevs_add kernel/locking/lockdep.c:3300 [inline]
validate_chain kernel/locking/lockdep.c:3924 [inline]
__lock_acquire+0x1492/0x1ec0 kernel/locking/lockdep.c:5253
lock_acquire kernel/locking/lockdep.c:5886 [inline]
lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5843
__fs_reclaim_acquire mm/page_alloc.c:4375 [inline]
fs_reclaim_acquire+0xc4/0x100 mm/page_alloc.c:4389
might_alloc include/linux/sched/mm.h:314 [inline]
slab_pre_alloc_hook mm/slub.c:4539 [inline]
slab_alloc_node mm/slub.c:4899 [inline]
kmem_cache_alloc_noprof+0x63/0x6a0 mm/slub.c:4933
__radix_tree_preload+0x333/0x820 lib/radix-tree.c:338
blkg_conf_prep+0x489/0xf10 block/blk-cgroup.c:885
tg_set_conf.isra.0+0x1ed/0x500 block/blk-throttle.c:1373
cgroup_file_write+0x1e6/0x8a0 kernel/cgroup/cgroup.c:4412
kernfs_fop_write_iter+0x3e0/0x5f0 fs/kernfs/file.c:345
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x6af/0x1050 fs/read_write.c:687
ksys_write+0x12a/0x250 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
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)#2 --> &q->blkcg_mutex
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&q->blkcg_mutex);
lock(&q->q_usage_counter(io)#2);
lock(&q->blkcg_mutex);
lock(fs_reclaim);
*** DEADLOCK ***
locks held by syz.3.20/6039: 6, last CPU#3:
#0: ffff88802339cd30 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2aa/0x380 fs/file.c:1259
#1: ffff888037ab0460 (sb_writers#11){.+.+}-{0:0}, at: ksys_write+0x12a/0x250 fs/read_write.c:739
#2: ffff88802f2bbc80 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x2c2/0x5f0 fs/kernfs/file.c:336
#3: ffff88802a08a5a8 (kn->active#66){.+.+}-{0:0}, at: kernfs_get_active_of fs/kernfs/file.c:73 [inline]
#3: ffff88802a08a5a8 (kn->active#66){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x332/0x5f0 fs/kernfs/file.c:337
#4: ffff888028badf80 (&q->rq_qos_mutex){+.+.}-{4:4}, at: blkg_conf_open_bdev+0x253/0x410 block/blk-cgroup.c:806
#5: ffff888028bae100 (&q->blkcg_mutex){+.+.}-{4:4}, at: blkg_conf_prep+0xbc/0xf10 block/blk-cgroup.c:849
stack backtrace:
CPU: 3 UID: 0 PID: 6039 Comm: syz.3.20 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
print_circular_bug.cold+0x178/0x1be kernel/locking/lockdep.c:2059
check_noncircular+0x146/0x160 kernel/locking/lockdep.c:2191
check_prev_add+0xeb/0xe60 kernel/locking/lockdep.c:3181
check_prevs_add kernel/locking/lockdep.c:3300 [inline]
validate_chain kernel/locking/lockdep.c:3924 [inline]
__lock_acquire+0x1492/0x1ec0 kernel/locking/lockdep.c:5253
lock_acquire kernel/locking/lockdep.c:5886 [inline]
lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5843
__fs_reclaim_acquire mm/page_alloc.c:4375 [inline]
fs_reclaim_acquire+0xc4/0x100 mm/page_alloc.c:4389
might_alloc include/linux/sched/mm.h:314 [inline]
slab_pre_alloc_hook mm/slub.c:4539 [inline]
slab_alloc_node mm/slub.c:4899 [inline]
kmem_cache_alloc_noprof+0x63/0x6a0 mm/slub.c:4933
__radix_tree_preload+0x333/0x820 lib/radix-tree.c:338
blkg_conf_prep+0x489/0xf10 block/blk-cgroup.c:885
tg_set_conf.isra.0+0x1ed/0x500 block/blk-throttle.c:1373
cgroup_file_write+0x1e6/0x8a0 kernel/cgroup/cgroup.c:4412
kernfs_fop_write_iter+0x3e0/0x5f0 fs/kernfs/file.c:345
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x6af/0x1050 fs/read_write.c:687
ksys_write+0x12a/0x250 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f87a939e0d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f87aa1ff028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f87a9625fa0 RCX: 00007f87a939e0d9
RDX: 000000000000006a RSI: 00002000000000c0 RDI: 0000000000000005
RBP: 00007f87a9435024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f87a9626038 R14: 00007f87a9625fa0 R15: 00007ffd6065d3e8
</TASK>
---
If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.
prev parent reply other threads:[~2026-08-21 9:20 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-15 14:47 [syzbot] [block?] [cgroups?] possible deadlock in blkg_conf_prep syzbot
2026-08-21 9:20 ` syzbot [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=6a881867.dbb3a75c.13dd47.000f.GAE@google.com \
--to=syzbot+144a1c0e22e53a08ef5a@syzkaller.appspotmail.com \
--cc=axboe@kernel.dk \
--cc=cgroups@vger.kernel.org \
--cc=josef@toxicpanda.com \
--cc=linux-block@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=syzkaller-bugs@googlegroups.com \
--cc=tj@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.