* [syzbot] [block?] [cgroups?] possible deadlock in blkg_conf_prep
@ 2026-08-15 14:47 syzbot
2026-08-21 9:20 ` syzbot
0 siblings, 1 reply; 2+ messages in thread
From: syzbot @ 2026-08-15 14:47 UTC (permalink / raw)
To: axboe, cgroups, josef, linux-block, linux-kernel, syzkaller-bugs,
tj
Hello,
syzbot found the following issue on:
HEAD commit: 3d08ff75a47a Add linux-next specific files for 20260810
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10427879580000
kernel config: https://syzkaller.appspot.com/x/.config?x=5ba0556605934602
dashboard link: https://syzkaller.appspot.com/bug?extid=144a1c0e22e53a08ef5a
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
Unfortunately, I don't have any reproducer for this issue yet.
Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/227afac921c2/disk-3d08ff75.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/236f7cfe0ac3/vmlinux-3d08ff75.xz
kernel image: https://storage.googleapis.com/syzbot-assets/aea89831fd96/bzImage-3d08ff75.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 Tainted: G L
------------------------------------------------------
syz.6.2908/17662 is trying to acquire lock:
ffffffff8ec88800 (fs_reclaim){+.+.}-{0:0}, at: might_alloc include/linux/sched/mm.h:316 [inline]
ffffffff8ec88800 (fs_reclaim){+.+.}-{0:0}, at: slab_pre_alloc_hook mm/slub.c:4636 [inline]
ffffffff8ec88800 (fs_reclaim){+.+.}-{0:0}, at: slab_alloc_node mm/slub.c:4974 [inline]
ffffffff8ec88800 (fs_reclaim){+.+.}-{0:0}, at: kmem_cache_alloc_noprof+0x5f/0x600 mm/slub.c:5010
but task is already holding lock:
ffff888026b33bc0 (&q->blkcg_mutex){+.+.}-{4:4}, at: blkg_conf_prep+0xa3/0xd80 block/blk-cgroup.c:846
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&q->blkcg_mutex){+.+.}-{4:4}:
__mutex_lock_common kernel/locking/mutex.c:646 [inline]
__mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
blkcg_activate_policy+0x1cb/0x8e0 block/blk-cgroup.c:1595
blk_throtl_init+0x2f7/0x420 block/blk-throttle.c:1339
tg_set_conf+0x1db/0x4e0 block/blk-throttle.c:1368
cgroup_file_write+0x331/0x8f0 kernel/cgroup/cgroup.c:4412
kernfs_fop_write_iter+0x3a4/0x540 fs/kernfs/file.c:345
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x612/0xba0 fs/read_write.c:687
ksys_write+0x150/0x270 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x510 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
-> #1 (&q->q_usage_counter(io)#17){++++}-{0:0}:
blk_alloc_queue+0x544/0x690 block/blk-core.c:504
blk_mq_alloc_queue block/blk-mq.c:4420 [inline]
__blk_mq_alloc_disk+0x194/0x390 block/blk-mq.c:4467
loop_add+0x480/0xb40 drivers/block/loop.c:2071
loop_init+0xd7/0x160 drivers/block/loop.c:2310
do_one_initcall+0x250/0x870 init/main.c:1359
do_initcall_level+0x10a/0x1a0 init/main.c:1421
do_initcalls+0x59/0xa0 init/main.c:1437
kernel_init_freeable+0x29d/0x3e0 init/main.c:1672
kernel_init+0x22/0x1d0 init/main.c:1562
ret_from_fork+0x514/0xb70 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 kernel/locking/lockdep.c:3181 [inline]
check_prevs_add kernel/locking/lockdep.c:3300 [inline]
validate_chain kernel/locking/lockdep.c:3924 [inline]
__lock_acquire+0x15ff/0x2e50 kernel/locking/lockdep.c:5254
lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5908
__fs_reclaim_acquire mm/page_alloc.c:4375 [inline]
fs_reclaim_acquire+0x71/0x100 mm/page_alloc.c:4389
might_alloc include/linux/sched/mm.h:316 [inline]
slab_pre_alloc_hook mm/slub.c:4636 [inline]
slab_alloc_node mm/slub.c:4974 [inline]
kmem_cache_alloc_noprof+0x5f/0x600 mm/slub.c:5010
__radix_tree_preload+0x2df/0x870 lib/radix-tree.c:338
blkg_conf_prep+0x621/0xd80 block/blk-cgroup.c:882
tg_set_conf+0x20b/0x4e0 block/blk-throttle.c:1373
cgroup_file_write+0x331/0x8f0 kernel/cgroup/cgroup.c:4412
kernfs_fop_write_iter+0x3a4/0x540 fs/kernfs/file.c:345
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x612/0xba0 fs/read_write.c:687
ksys_write+0x150/0x270 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x510 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)#17 --> &q->blkcg_mutex
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&q->blkcg_mutex);
lock(&q->q_usage_counter(io)#17);
lock(&q->blkcg_mutex);
lock(fs_reclaim);
*** DEADLOCK ***
locks held by syz.6.2908/17662: 6, last CPU#0:
#0: ffff888035aab0b0 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x246/0x320 fs/file.c:1259
#1: ffff8880767f2460 (sb_writers#10){.+.+}-{0:0}, at: file_start_write include/linux/fs.h:2754 [inline]
#1: ffff8880767f2460 (sb_writers#10){.+.+}-{0:0}, at: vfs_write+0x22b/0xba0 fs/read_write.c:683
#2: ffff888068348480 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x1d8/0x540 fs/kernfs/file.c:336
#3: ffff888064aeab48 (kn->active#70){.+.+}-{0:0}, at: kernfs_get_active_of fs/kernfs/file.c:73 [inline]
#3: ffff888064aeab48 (kn->active#70){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x22b/0x540 fs/kernfs/file.c:337
#4: ffff888026b33a40 (&q->rq_qos_mutex){+.+.}-{4:4}, at: blkg_conf_open_bdev+0x2a8/0x3c0 block/blk-cgroup.c:803
#5: ffff888026b33bc0 (&q->blkcg_mutex){+.+.}-{4:4}, at: blkg_conf_prep+0xa3/0xd80 block/blk-cgroup.c:846
stack backtrace:
CPU: 0 UID: 0 PID: 17662 Comm: syz.6.2908 Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2059
check_noncircular+0x12f/0x150 kernel/locking/lockdep.c:2191
check_prev_add kernel/locking/lockdep.c:3181 [inline]
check_prevs_add kernel/locking/lockdep.c:3300 [inline]
validate_chain kernel/locking/lockdep.c:3924 [inline]
__lock_acquire+0x15ff/0x2e50 kernel/locking/lockdep.c:5254
lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5908
__fs_reclaim_acquire mm/page_alloc.c:4375 [inline]
fs_reclaim_acquire+0x71/0x100 mm/page_alloc.c:4389
might_alloc include/linux/sched/mm.h:316 [inline]
slab_pre_alloc_hook mm/slub.c:4636 [inline]
slab_alloc_node mm/slub.c:4974 [inline]
kmem_cache_alloc_noprof+0x5f/0x600 mm/slub.c:5010
__radix_tree_preload+0x2df/0x870 lib/radix-tree.c:338
blkg_conf_prep+0x621/0xd80 block/blk-cgroup.c:882
tg_set_conf+0x20b/0x4e0 block/blk-throttle.c:1373
cgroup_file_write+0x331/0x8f0 kernel/cgroup/cgroup.c:4412
kernfs_fop_write_iter+0x3a4/0x540 fs/kernfs/file.c:345
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x612/0xba0 fs/read_write.c:687
ksys_write+0x150/0x270 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x510 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f1a06d9e0d9
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:00007f1a07bcb028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f1a07025fa0 RCX: 00007f1a06d9e0d9
RDX: 000000000000006a RSI: 00002000000000c0 RDI: 0000000000000008
RBP: 00007f1a06e35024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f1a07026038 R14: 00007f1a07025fa0 R15: 00007ffdd2d5d118
</TASK>
---
This report is generated by a bot. It may contain errors.
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syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
If the report is already addressed, let syzbot know by replying with:
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If you want to overwrite report's subsystems, reply with:
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If the report is a duplicate of another one, reply with:
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If you want to undo deduplication, reply with:
#syz undup
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [syzbot] [block?] [cgroups?] possible deadlock in blkg_conf_prep
2026-08-15 14:47 [syzbot] [block?] [cgroups?] possible deadlock in blkg_conf_prep syzbot
@ 2026-08-21 9:20 ` syzbot
0 siblings, 0 replies; 2+ messages in thread
From: syzbot @ 2026-08-21 9:20 UTC (permalink / raw)
To: axboe, cgroups, josef, linux-block, linux-kernel, syzkaller-bugs,
tj
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.
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2026-08-21 9:20 UTC | newest]
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2026-08-15 14:47 [syzbot] [block?] [cgroups?] possible deadlock in blkg_conf_prep syzbot
2026-08-21 9:20 ` syzbot
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