Re: [syzbot] [mm?] INFO: rcu detected stall in exit_to_user_mode_loop

From: syzbot

Date: Mon Aug 31 2026 - 07:13:14 EST

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

2.992004][ T1] Mandatory Access Control activated.
[ 2.993509][ T1] pnp: PnP ACPI init
[ 3.023863][ T1] pnp: PnP ACPI: found 6 devices
[ 3.105394][ T1] clocksource: acpi_pm: mask: 0xffffff max_cycles: 0xffffff, max_idle_ns: 2085701024 ns
[ 3.113910][ T1] NET: Registered PF_INET protocol family
[ 3.116610][ T1] IP idents hash table entries: 131072 (order: 8, 1048576 bytes, vmalloc)
[ 3.128889][ T1] tcp_listen_portaddr_hash hash table entries: 4096 (order: 7, 294912 bytes, vmalloc)
[ 3.130899][ T1] Table-perturb hash table entries: 65536 (order: 6, 262144 bytes, vmalloc)
[ 3.133075][ T1] TCP established hash table entries: 65536 (order: 7, 524288 bytes, vmalloc)
[ 3.144444][ T1] TCP bind hash table entries: 65536 (order: 12, 9437184 bytes, vmalloc hugepage)
[ 3.151706][ T1] TCP: Hash tables configured (established 65536 bind 65536)
[ 3.155107][ T1] MPTCP token hash table entries: 8192 (order: 8, 720896 bytes, vmalloc)
[ 3.158207][ T1] UDP hash table entries: 4096 (order: 8, 1048576 bytes, vmalloc)
[ 3.161213][ T1] NET: Registered PF_UNIX/PF_LOCAL protocol family
[ 3.166692][ T1] RPC: Registered named UNIX socket transport module.
[ 3.167740][ T1] RPC: Registered udp transport module.
[ 3.168610][ T1] RPC: Registered tcp transport module.
[ 3.169492][ T1] RPC: Registered tcp-with-tls transport module.
[ 3.170446][ T1] RPC: Registered tcp NFSv4.1 backchannel transport module.
[ 3.187812][ T1] NET: Registered PF_XDP protocol family
[ 3.189221][ T1] pci_bus 0000:00: resource 4 [io 0x0000-0x0cf7 window]
[ 3.190367][ T1] pci_bus 0000:00: resource 5 [io 0x0d00-0xffff window]
[ 3.191848][ T1] pci_bus 0000:00: resource 6 [mem 0x000a0000-0x000bffff window]
[ 3.193134][ T1] pci_bus 0000:00: resource 7 [mem 0xc0000000-0xfebfefff window]
[ 3.195979][ T1] pci 0000:00:00.0: Limiting direct PCI/PCI transfers
[ 3.197272][ T1] PCI: CLS 0 bytes, default 64
[ 3.198358][ T1] PCI-DMA: Using software bounce buffering for IO (SWIOTLB)
[ 3.199469][ T1] software IO TLB: mapped [mem 0x00000000b4400000-0x00000000b8400000] (64MB)
[ 3.201954][ T1] ACPI: bus type thunderbolt registered
[ 3.211140][ T1] RAPL PMU: API unit is 2^-32 Joules, 0 fixed counters, 10737418240 ms ovfl timer
[ 3.216011][ T61] kworker/u8:1 (61) used greatest stack depth: 27880 bytes left
[ 3.236610][ T1] kvm_amd: CPU 0 isn't AMD or Hygon
[ 3.236649][ T1] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x285d5483c8d, max_idle_ns: 440795228937 ns
[ 3.239738][ T1] clocksource: Switched to clocksource tsc
[ 3.247879][ T67] kworker/u8:3 (67) used greatest stack depth: 27160 bytes left
[ 3.274579][ T1] Initialise system trusted keyrings
[ 3.278300][ T1] workingset: timestamp_bits=40 (anon: 35) max_order=21 bucket_order=0 (anon: 0)
[ 3.286066][ T1] DLM installed
[ 3.291910][ T1] squashfs: version 4.0 (2009/01/31) Phillip Lougher
[ 3.299536][ T1] NFS: Registering the id_resolver key type
[ 3.299710][ T1] Key type id_resolver registered
[ 3.299721][ T1] Key type id_legacy registered
[ 3.299981][ T1] nfs4filelayout_init: NFSv4 File Layout Driver Registering...
[ 3.300091][ T1] nfs4flexfilelayout_init: NFSv4 Flexfile Layout Driver Registering...
[ 3.304030][ T1] smbdirect: subsystem loading...
[ 3.312200][ T1] smbdirect: subsystem loaded
[ 3.342789][ T1] Key type cifs.spnego registered
[ 3.343043][ T1] Key type cifs.idmap registered
[ 3.346819][ T1] ntfs3: Enabled Linux POSIX ACLs support
[ 3.346847][ T1] ntfs3: Read-only LZX/Xpress compression included
[ 3.347118][ T1] jffs2: version 2.2. (NAND) (SUMMARY) © 2001-2006 Red Hat, Inc.
[ 3.349230][ T1] romfs: ROMFS MTD (C) 2007 Red Hat, Inc.
[ 3.349425][ T1] QNX4 filesystem 0.2.3 registered.
[ 3.349531][ T1] qnx6: QNX6 filesystem 1.0.0 registered.
[ 3.350763][ T1] fuse: init (API version 7.46)
[ 3.354681][ T1] orangefs_debugfs_init: called with debug mask: :none: :0:
[ 3.355943][ T1] orangefs_init: module version upstream loaded
[ 3.356821][ T1] JFS: nTxBlock = 8192, nTxLock = 65536
[ 3.369044][ T1] SGI XFS with ACLs, security attributes, realtime, scrub, repair, quota, no debug enabled
[ 3.378590][ T1] 9p: Installing v9fs 9p2000 file system support
[ 3.380623][ T1] NILFS version 2 loaded
[ 3.381303][ T1] befs: version: 0.9.3
[ 3.382815][ T1] ocfs2: Registered cluster interface o2cb
[ 3.384592][ T1] ocfs2: Registered cluster interface user
[ 3.386061][ T1] OCFS2 User DLM kernel interface loaded
[ 3.408890][ T1] gfs2: GFS2 installed
[ 3.423843][ T1] ceph: loaded (mds proto 32)
[ 3.437963][ T1] NET: Registered PF_ALG protocol family
[ 3.439495][ T1] async_tx: api initialized (async)
[ 3.440455][ T1] Key type asymmetric registered
[ 3.441477][ T1] Asymmetric key parser 'x509' registered
[ 3.442418][ T1] Asymmetric key parser 'pkcs8' registered
[ 3.443295][ T1] Key type pkcs7_test registered
[ 3.444626][ T1] Block layer SCSI generic (bsg) driver version 0.4 loaded (major 239)
[ 3.448227][ T1] io scheduler mq-deadline registered
[ 3.449247][ T1] io scheduler kyber registered
[ 3.450679][ T1] io scheduler bfq registered
[ 3.452609][ T1] raid6: skipped pq benchmark and selected avx512x4
[ 3.470190][ T1] input: Power Button as /devices/platform/LNXPWRBN:00/input/input0
[ 3.473842][ T1] ACPI: button: Power Button [PWRF]
[ 3.477749][ T1] input: Sleep Button as /devices/platform/LNXSLPBN:00/input/input1
[ 3.480757][ T1] ACPI: button: Sleep Button [SLPF]
[ 3.501883][ T1] ioatdma: Intel(R) QuickData Technology Driver 5.00
[ 3.538986][ T1] ACPI: \_SB_.LNKC: Enabled at IRQ 11
[ 3.540120][ T1] virtio-pci 0000:00:03.0: virtio_pci: leaving for legacy driver
[ 3.573242][ T1] ACPI: \_SB_.LNKD: Enabled at IRQ 10
[ 3.574257][ T1] virtio-pci 0000:00:04.0: virtio_pci: leaving for legacy driver
[ 3.611123][ T1] ACPI: \_SB_.LNKB: Enabled at IRQ 10
[ 3.612311][ T1] virtio-pci 0000:00:06.0: virtio_pci: leaving for legacy driver
[ 4.026288][ T629] kworker/u8:6 (629) used greatest stack depth: 26904 bytes left
[ 4.103079][ T1] N_HDLC line discipline registered with maxframe=4096
[ 4.105350][ T1] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
[ 4.114516][ T1] 00:02: ttyS0 I/O:0x3f8 (irq = 4, base_baud = 115200) is a 16550A
[ 4.129686][ T1] 00:03: ttyS1 I/O:0x2f8 (irq = 3, base_baud = 115200) is a 16550A
[ 4.144792][ T1] 00:04: ttyS2 I/O:0x3e8 (irq = 6, base_baud = 115200) is a 16550A
[ 4.169157][ T1] 00:05: ttyS3 I/O:0x2e8 (irq = 7, base_baud = 115200) is a 16550A
[ 4.206372][ T1] Non-volatile memory driver v1.3
[ 4.228763][ T1] usbcore: registered new interface driver xillyusb
[ 4.234701][ T1] ACPI: bus type drm_connector registered
[ 4.245698][ T1] [drm] Initialized vgem 1.0.0 for vgem on minor 0
[ 4.252028][ T1] ------------[ cut here ]------------
[ 4.252067][ T1] [PLANE:35:plane-0] pixel format with alpha exposed but blend mode not setup
[ 4.252085][ T1] WARNING: drivers/gpu/drm/drm_mode_config.c:872 at drm_mode_config_validate+0xfb4/0x1be0, CPU#0: swapper/0/1
[ 4.256488][ T1] Modules linked in:
[ 4.257080][ T1] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted syzkaller #0 PREEMPT(full)
[ 4.258850][ T1] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/05/2026
[ 4.260606][ T1] RIP: 0010:drm_mode_config_validate+0xfbb/0x1be0
[ 4.261603][ T1] Code: 00 49 8b 57 18 48 89 f8 48 c1 e8 03 0f b6 04 28 84 c0 74 08 3c 03 0f 8e 00 0b 00 00 48 8d 3d 5c 1f 7e 0b 41 8b b7 c8 00 00 00 <67> 48 0f b9 3a e9 fa fd ff ff 48 8b 5c 24 20 e8 d1 aa 39 fc 48 8d
[ 4.264757][ T1] RSP: 0000:ffffc90000067c18 EFLAGS: 00010246
[ 4.265631][ T1] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[ 4.266846][ T1] RDX: ffff88802708ff80 RSI: 0000000000000023 RDI: ffffffff91504ba0
[ 4.268445][ T1] RBP: dffffc0000000000 R08: 0000000000000001 R09: 0000000000000000
[ 4.269744][ T1] R10: 0000000000000001 R11: 0000000000000000 R12: ffffed1004ea1e23
[ 4.270879][ T1] R13: ffffed1004ea1e24 R14: 0000000000000001 R15: ffff88802750f028
[ 4.272125][ T1] FS: 0000000000000000(0000) GS:ffff888123b65000(0000) knlGS:0000000000000000
[ 4.273520][ T1] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 4.274598][ T1] CR2: ffff88823ffff000 CR3: 000000000eb96000 CR4: 00000000003526f0
[ 4.275805][ T1] Call Trace:
[ 4.276304][ T1] <TASK>
[ 4.276744][ T1] drm_dev_register+0x56e/0x7b0
[ 4.277432][ T1] vkms_create+0x491/0x5b0
[ 4.278450][ T1] ? __pfx_vkms_init+0x10/0x10
[ 4.279187][ T1] vkms_init+0x98/0xe0
[ 4.279870][ T1] do_one_initcall+0x11c/0x6f0
[ 4.280657][ T1] ? __pfx_do_one_initcall+0x10/0x10
[ 4.281473][ T1] ? kernel_init_freeable+0x4ca/0x7b0
[ 4.282372][ T1] kernel_init_freeable+0x6ea/0x7b0
[ 4.283205][ T1] ? __pfx_kernel_init+0x10/0x10
[ 4.283952][ T1] kernel_init+0x21/0x1e0
[ 4.284623][ T1] ? __pfx_kernel_init+0x10/0x10
[ 4.285469][ T1] ret_from_fork+0x730/0xd60
[ 4.286179][ T1] ? __pfx_ret_from_fork+0x10/0x10
[ 4.286964][ T1] ? __switch_to+0x800/0x10f0
[ 4.288061][ T1] ? __pfx_kernel_init+0x10/0x10
[ 4.288883][ T1] ret_from_fork_asm+0x1a/0x30
[ 4.289745][ T1] </TASK>
[ 4.290229][ T1] Kernel panic - not syncing: kernel: panic_on_warn set ...
[ 4.291328][ T1] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted syzkaller #0 PREEMPT(full)
[ 4.292619][ T1] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/05/2026
[ 4.294006][ T1] Call Trace:
[ 4.294490][ T1] <TASK>
[ 4.294982][ T1] dump_stack_lvl+0x100/0x190
[ 4.295673][ T1] vpanic+0x553/0x970
[ 4.296729][ T1] ? __pfx_vpanic+0x10/0x10
[ 4.297560][ T1] panic+0xd1/0xe0
[ 4.297698][ T1] ? __pfx_panic+0x10/0x10
[ 4.297698][ T1] check_panic_on_warn.cold+0x19/0x34
[ 4.297698][ T1] ? drm_mode_config_validate+0xfb4/0x1be0
[ 4.297698][ T1] __warn.cold+0x191/0x318
[ 4.297698][ T1] __report_bug+0x30f/0x440
[ 4.297698][ T1] ? drm_mode_config_validate+0xfb4/0x1be0
[ 4.297698][ T1] ? __pfx___report_bug+0x10/0x10
[ 4.297698][ T1] ? reacquire_held_locks+0xdd/0x1f0
[ 4.297698][ T1] report_bug_entry+0xe2/0x290
[ 4.297698][ T1] ? drm_mode_config_validate+0xfbb/0x1be0
[ 4.297698][ T1] handle_bug+0x1cd/0x2a0
[ 4.297698][ T1] exc_invalid_op+0x17/0x50
[ 4.297698][ T1] asm_exc_invalid_op+0x1a/0x20
[ 4.297698][ T1] RIP: 0010:drm_mode_config_validate+0xfbb/0x1be0
[ 4.297698][ T1] Code: 00 49 8b 57 18 48 89 f8 48 c1 e8 03 0f b6 04 28 84 c0 74 08 3c 03 0f 8e 00 0b 00 00 48 8d 3d 5c 1f 7e 0b 41 8b b7 c8 00 00 00 <67> 48 0f b9 3a e9 fa fd ff ff 48 8b 5c 24 20 e8 d1 aa 39 fc 48 8d
[ 4.297698][ T1] RSP: 0000:ffffc90000067c18 EFLAGS: 00010246
[ 4.297698][ T1] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[ 4.297698][ T1] RDX: ffff88802708ff80 RSI: 0000000000000023 RDI: ffffffff91504ba0
[ 4.297698][ T1] RBP: dffffc0000000000 R08: 0000000000000001 R09: 0000000000000000
[ 4.297698][ T1] R10: 0000000000000001 R11: 0000000000000000 R12: ffffed1004ea1e23
[ 4.297698][ T1] R13: ffffed1004ea1e24 R14: 0000000000000001 R15: ffff88802750f028
[ 4.297698][ T1] ? drm_mode_config_validate+0xf76/0x1be0
[ 4.297698][ T1] drm_dev_register+0x56e/0x7b0
[ 4.297698][ T1] vkms_create+0x491/0x5b0
[ 4.297698][ T1] ? __pfx_vkms_init+0x10/0x10
[ 4.297698][ T1] vkms_init+0x98/0xe0
[ 4.297698][ T1] do_one_initcall+0x11c/0x6f0
[ 4.297698][ T1] ? __pfx_do_one_initcall+0x10/0x10
[ 4.297698][ T1] ? kernel_init_freeable+0x4ca/0x7b0
[ 4.297698][ T1] kernel_init_freeable+0x6ea/0x7b0
[ 4.297698][ T1] ? __pfx_kernel_init+0x10/0x10
[ 4.297698][ T1] kernel_init+0x21/0x1e0
[ 4.297698][ T1] ? __pfx_kernel_init+0x10/0x10
[ 4.297698][ T1] ret_from_fork+0x730/0xd60
[ 4.297698][ T1] ? __pfx_ret_from_fork+0x10/0x10
[ 4.297698][ T1] ? __switch_to+0x800/0x10f0
[ 4.297698][ T1] ? __pfx_kernel_init+0x10/0x10
[ 4.297698][ T1] ret_from_fork_asm+0x1a/0x30
[ 4.297698][ T1] </TASK>
[ 4.297698][ T1] Kernel Offset: disabled
[ 4.297698][ T1] Rebooting in 86400 seconds..

syzkaller build log:
go env (err=<nil>)
AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE='auto'
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/syzkaller/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/syzkaller/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1798266567=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/syzkaller/jobs-2/linux/gopath/src/github.com/google/syzkaller/go.mod'
GOMODCACHE='/syzkaller/jobs-2/linux/gopath/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/syzkaller/jobs-2/linux/gopath'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/syzkaller/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26.0'
GOWORK=''
PKG_CONFIG='pkg-config'

git status (err=<nil>)
HEAD detached at 4cb2b096987
nothing to commit, working tree clean

tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
Makefile:31: run command via tools/syz-env for best compatibility, see:
Makefile:32: https://github.com/google/syzkaller/blob/master/docs/contributing.md#using-syz-env
go list -f '{{.Stale}}' -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=4cb2b096987c8122dc6b37eb51200b1ca7991e0f -X github.com/google/syzkaller/prog.gitRevisionDate=20260727-130118" ./sys/syz-sysgen | grep -q false || go install -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=4cb2b096987c8122dc6b37eb51200b1ca7991e0f -X github.com/google/syzkaller/prog.gitRevisionDate=20260727-130118" ./sys/syz-sysgen
make .descriptions
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
Makefile:31: run command via tools/syz-env for best compatibility, see:
Makefile:32: https://github.com/google/syzkaller/blob/master/docs/contributing.md#using-syz-env
bin/syz-sysgen
touch .descriptions
GOOS=linux GOARCH=amd64 go build -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=4cb2b096987c8122dc6b37eb51200b1ca7991e0f -X github.com/google/syzkaller/prog.gitRevisionDate=20260727-130118" -o ./bin/linux_amd64/syz-execprog github.com/google/syzkaller/tools/syz-execprog
mkdir -p ./bin/linux_amd64
g++ -o ./bin/linux_amd64/syz-executor executor/executor.cc \
-m64 -O2 -pthread -Wall -Werror -Wparentheses -Wunused-const-variable -Wframe-larger-than=16384 -Wno-stringop-overflow -Wno-array-bounds -Wno-format-overflow -Wno-unused-but-set-variable -Wno-unused-command-line-argument -static-pie -std=c++17 -I. -Iexecutor/_include -DGOOS_linux=1 -DGOARCH_amd64=1 \
-DHOSTGOOS_linux=1 -DGIT_REVISION=\"4cb2b096987c8122dc6b37eb51200b1ca7991e0f\"
/usr/bin/ld: /tmp/ccZHlTeq.o: in function `Connection::Connect(char const*, char const*)':
executor.cc:(.text._ZN10Connection7ConnectEPKcS1_[_ZN10Connection7ConnectEPKcS1_]+0x386): warning: Using 'gethostbyname' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
./tools/check-syzos.sh 2>/dev/null

Error text is too large and was truncated, full error text is at:
https://syzkaller.appspot.com/x/error.txt?x=14ce4349580000

Tested on:

commit: a8455260 ipvlan: unregister upper devices outside pnod..
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git
kernel config: https://syzkaller.appspot.com/x/.config?x=9cc692b98d9b4fdb
dashboard link: https://syzkaller.appspot.com/bug?extid=2642f347f7309b4880dc
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
patch: https://syzkaller.appspot.com/x/patch.diff?x=11aef379580000