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Wed, 7 Jan 2026 05:36:55 +0000 Received: from KL1PR03MB8800.apcprd03.prod.outlook.com ([fe80::95dd:dd8:3bbf:2c16]) by KL1PR03MB8800.apcprd03.prod.outlook.com ([fe80::95dd:dd8:3bbf:2c16%6]) with mapi id 15.20.9478.004; Wed, 7 Jan 2026 05:36:55 +0000 From: "WangzXD0325@outlook.com" To: "linux-fsdevel@vger.kernel.org" CC: "linux-kernel@vger.kernel.org" , "rcu@vger.kernel.org" , "syzkaller-bugs@googlegroups.com" , "akpm@linux-foundation.org" , "paulmck@kernel.org" , "torvalds@linux-foundation.org" Subject: Subject: [BUG] rcu: stall detected in __dentry_kill during file teardown Thread-Topic: Subject: [BUG] rcu: stall detected in __dentry_kill during file teardown Thread-Index: AQHcf5eY5TLNSudu8kCb3bY1LT8FUw== Date: Wed, 7 Jan 2026 05:36:54 +0000 Message-ID: Accept-Language: zh-CN, en-US Content-Language: zh-CN X-MS-Has-Attach: X-MS-TNEF-Correlator: msip_labels: x-ms-exchange-messagesentrepresentingtype: 1 x-ms-publictraffictype: Email x-ms-traffictypediagnostic: KL1PR03MB8800:EE_|TY1PPF6FE718230:EE_ x-ms-office365-filtering-correlation-id: 498bcee2-00be-4485-a48a-08de4daec48e x-ms-exchange-slblob-mailprops: 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 x-microsoft-antispam: BCL:0;ARA:14566002|31061999003|12121999013|19110799012|15080799012|8062599012|8060799015|15030799006|461199028|440099028|40105399003|52005399003|3412199025|102099032|3430499032|26104999006|12091999003; 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charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-OriginatorOrg: outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: KL1PR03MB8800.apcprd03.prod.outlook.com X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-CrossTenant-Network-Message-Id: 498bcee2-00be-4485-a48a-08de4daec48e X-MS-Exchange-CrossTenant-originalarrivaltime: 07 Jan 2026 05:36:54.9980 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa X-MS-Exchange-CrossTenant-rms-persistedconsumerorg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-Transport-CrossTenantHeadersStamped: TY1PPF6FE718230 Hello,=0A= =0A= I am reporting an RCU stall detected in __dentry_kill(), which was=0A= observed during syzkaller-style fuzz testing. The stall eventually=0A= leads to prolonged RCU grace-period starvation and system instability.=0A= =0A= =3D=3D=3D Summary =3D=3D=3D=0A= The kernel reports an RCU preempt stall involving __dentry_kill()=0A= during file and dentry teardown. One CPU remains stuck in a filesystem=0A= cleanup path while holding locks, preventing RCU grace periods from=0A= completing.=0A= =0A= The stall is detected reliably under syzkaller fuzzing.=0A= =0A= =3D=3D=3D Environment =3D=3D=3D=0A= Kernel: 6.18.0 (locally built)=0A= Config: PREEMPT(full)=0A= Arch: x86_64=0A= Hardware: QEMU Standard PC (i440FX + PIIX)=0A= Workload: syzkaller (syz-executor)=0A= =0A= =3D=3D=3D Triggering context =3D=3D=3D=0A= The stall is detected while a syzkaller executor process is exiting and=0A= releasing file descriptors. The problematic task is in process context=0A= during file teardown:=0A= =0A= =A0 do_exit=0A= =A0 __fput=0A= =A0 dput=0A= =A0 __dentry_kill=0A= =A0 dentry_unlink_inode=0A= =A0 iput=0A= =A0 evict=0A= =A0 shmem_evict_inode=0A= =A0 shmem_truncate_range=0A= =0A= At the same time, other CPUs are observed handling DRM-related atomic=0A= operations and vblank timers.=0A= =0A= =3D=3D=3D Warning details =3D=3D=3D=0A= The kernel reports:=0A= =0A= =A0 INFO: rcu detected stall in __dentry_kill=0A= =A0 rcu: rcu_preempt detected stalls on CPUs/tasks=0A= =0A= The stall persists for over 10k jiffies, triggering NMI backtraces on=0A= multiple CPUs.=0A= =0A= RCU also reports starvation of the grace-period kthread:=0A= =0A= =A0 rcu: rcu_preempt kthread starved for 10502 jiffies=0A= =0A= =3D=3D=3D Call trace =3D=3D=3D=0A= Stalled task backtrace (CPU 3):=0A= =0A= =A0 lock_release=0A= =A0 _raw_spin_unlock_irqrestore=0A= =A0 hrtimer_cancel=0A= =A0 drm_vblank_disable_and_save=0A= =A0 drm_crtc_vblank_off=0A= =A0 drm_atomic_helper_commit_modeset_disables=0A= =A0 drm_atomic_commit=0A= =A0 drm_mode_obj_set_property_ioctl=0A= =A0 drm_ioctl=0A= =A0 __do_sys_ioctl=0A= =0A= File teardown path involved in the stall:=0A= =0A= =A0 __dentry_kill=0A= =A0 dentry_unlink_inode=0A= =A0 iput=0A= =A0 evict=0A= =A0 shmem_evict_inode=0A= =A0 shmem_truncate_range=0A= =A0 __fput=0A= =A0 do_exit=0A= =0A= =3D=3D=3D Observations =3D=3D=3D=0A= The RCU stall occurs while tearing down dentries and inodes, suggesting=0A= that a filesystem cleanup path may be holding locks or disabling=0A= preemption for an excessive amount of time.=0A= =0A= The presence of DRM atomic commit and vblank timer activity in parallel=0A= may exacerbate scheduling delays, but the stall is ultimately detected=0A= in __dentry_kill().=0A= =0A= This points to a possible locking or preemption imbalance in dentry or=0A= inode teardown paths, potentially triggered by adversarial userspace=0A= behavior.=0A= =0A= =3D=3D=3D Reproducer =3D=3D=3D=0A= No standalone reproducer is available.=0A= The issue was observed during syzkaller-style fuzzing.=0A= =0A= =3D=3D=3D Expected behavior =3D=3D=3D=0A= RCU grace periods should complete without stalls during normal or=0A= malformed file teardown operations.=0A= =0A= =3D=3D=3D Actual behavior =3D=3D=3D=0A= RCU reports prolonged stalls in __dentry_kill(), leading to repeated=0A= NMI backtraces and RCU kthread starvation.=0A= =0A= =3D=3D=3D Notes =3D=3D=3D=0A= Additional logs, full kernel configuration, or syzkaller artifacts can=0A= be provided if needed.=0A= =0A= Reported-by:=0A= Zhi Wang=