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Thu, 05 Feb 2026 04:57:35 -0800 (PST) MIME-Version: 1.0 From: Zw Tang Date: Thu, 5 Feb 2026 20:57:24 +0800 X-Gm-Features: AZwV_QjDTHGRFN0So-_CCiElDZ_kjRQAvoGB44WIaGt4o6PEh2_5MuYnPtg2dJo Message-ID: Subject: [BUG] rcu detected stall in shmem_file_write_iter To: "linux-mm@kvack.org" , rcu@vger.kernel.org Cc: "hughd@google.com" , "akpm@linux-foundation.org" , "david@kernel.org" , chrisl@kernel.org, kasong@tencent.com, paulmck@kernel.org, frederic@kernel.org, "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: F389740009 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: rw68x74oyo7tmxhrngwia5ynxyfmbztf X-HE-Tag: 1770296256-986763 X-HE-Meta: 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 abdZQdkP ueucwnMA/IJH3Ldm84HaLmvxWPVrhnlacSbBcv8gtX7iq7YOWFdte9bl4AH7GEL1O8wzrIo7/7vjDRuQKavTaReYAa5748fzZXUH4qqM243/7O0paSxWAOaBzzrZrW8SPibCcgTcPUJXSuDColZed7d+ukqcNvObrvFu7MgfHy8xoPabw+0gydPn51VdULfq7Y729mJkU+pGJ79mUZcDT8mMfbEWQdoRDTXRUHhwN6xKTkkE52H97onf1gWp1L0wpnlnq69I54XGCXSiZEV7P4rCyHIeVG6uuf1CpWXOsY7YGsK3YA7OIRaTgJbkIMMGw6hOcXi+E7yR4EjxE2j0n6BK7tVSUvRTpJbrRrj6M7BD3pXxt0X1Arbu/EGsB9EyKwhKVuuIkYcGl3CUtxQ4Wd7USYxpwylh+Rm2jxjdA8Fiz6cuh3BfXrUj8jejzL21yBivqom/R0bdFRLJEAgvNCglyB4T9wdSYmwHK X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi, I am reporting a reproducible RCU stall observed on Linux 6.19.0-rc7, triggered by a syzkaller C reproducer. The stall is reported while a userspace task is executing the tmpfs (shmem) write path. The blocked task is a syz-executor process, and the RCU report consistently shows it running in the shmem write / folio allocation path for an extended period of time. The relevant call trace of the stalled task is: shmem_file_write_iter shmem_write_begin shmem_get_folio_gfp __folio_batch_add_and_move folio_batch_move_lru lru_add __mod_zone_page_state The kernel eventually reports: rcu: INFO: rcu_preempt detected stalls on CPUs/tasks This suggests that the task spends an excessive amount of time in the shmem write and folio/LRU accounting path, preventing the CPU from reaching a quiescent state and triggering the RCU stall detector. I am not yet certain whether the stall is caused by heavy memory pressure, LRU/zone accounting contention, or an unintended long-running critical section in the shmem write path, but the stall is consistently associated with shmem_file_write_iter(). Reproducer: C reproducer: https://pastebin.com/raw/AjQ5a5PL console output: https://pastebin.com/raw/FyBF1R7b kernel config: https://pastebin.com/raw/LwALTGZ5 Kernel: git tree: torvalds/linux HEAD commit: 63804fed149a6750ffd28610c5c1c98cce6bd377 kernel version: 6.19.0-rc7 (QEMU Ubuntu 24.10) rcu: INFO: rcu_preempt detected stalls on CPUs/tasks: rcu: Tasks blocked on level-0 rcu_node (CPUs 0-1): P51643 rcu: (detected by 1, t=100002 jiffies, g=470049, q=86 ncpus=2) task:syz.3.5719 state:R running task stack:25640 pid:51643 tgid:51627 ppid:49386 task_flags:0x400140 flags:0x00080012 Call Trace: sched_show_task kernel/sched/core.c:7821 [inline] sched_show_task+0x357/0x510 kernel/sched/core.c:7796 rcu_print_detail_task_stall_rnp kernel/rcu/tree_stall.h:292 [inline] print_other_cpu_stall kernel/rcu/tree_stall.h:681 [inline] check_cpu_stall kernel/rcu/tree_stall.h:856 [inline] rcu_pending kernel/rcu/tree.c:3667 [inline] rcu_sched_clock_irq+0x20ab/0x27e0 kernel/rcu/tree.c:2704 update_process_times+0xf4/0x160 kernel/time/timer.c:2474 tick_sched_handle kernel/time/tick-sched.c:298 [inline] tick_nohz_handler+0x504/0x720 kernel/time/tick-sched.c:319 __run_hrtimer kernel/time/hrtimer.c:1777 [inline] __hrtimer_run_queues+0x274/0x810 kernel/time/hrtimer.c:1841 hrtimer_interrupt+0x2f3/0x750 kernel/time/hrtimer.c:1903 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1045 [inline] __sysvec_apic_timer_interrupt+0x82/0x250 arch/x86/kernel/apic/apic.c:1062 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1056 [inline] sysvec_apic_timer_interrupt+0x6b/0x80 arch/x86/kernel/apic/apic.c:1056 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697 RIP: 0010:finish_task_switch+0x128/0x610 kernel/sched/core.c:5118 Code: 02 00 0f 85 67 04 00 00 49 8b 9c 24 98 0a 00 00 48 85 db 0f 85 70 03 00 00 4c 89 e7 e8 61 78 92 02 fb 65 48 8b 1d 68 51 5d 04 <48> 8d bb e0 0a 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 RSP: 0018:ffff88802d32f630 EFLAGS: 00000286 RAX: 0000000000000000 RBX: ffff888012496900 RCX: 0000000000000000 RDX: 0000000000000000 RSI: ffff888012496900 RDI: ffff88806d535b80 RBP: ffff88802d32f670 R08: 0000000000000000 R09: ffffffff817f85a5 R10: 0000000000000000 R11: 0000000000000000 R12: ffff88806d535b80 R13: ffff88800635c600 R14: ffff88800f630f00 R15: ffff888012497374 context_switch kernel/sched/core.c:5263 [inline] __schedule+0x1293/0x38c0 kernel/sched/core.c:6867 preempt_schedule_irq+0x49/0x70 kernel/sched/core.c:7194 irqentry_exit+0xc1/0x5a0 kernel/entry/common.c:216 asm_sysvec_irq_work+0x1a/0x20 arch/x86/include/asm/idtentry.h:733 RIP: 0010:__mod_zone_page_state+0x12/0xf0 mm/vmstat.c:347 Code: 89 ef e8 b1 53 18 00 e9 54 ff ff ff 66 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 b8 00 00 00 00 00 fc ff df 41 55 41 54 <55> 48 89 fd 48 83 c7 70 53 48 89 f9 48 c1 e9 03 48 83 ec 10 80 3c RSP: 0018:ffff88802d32f898 EFLAGS: 00000286 RAX: dffffc0000000000 RBX: ffff88800c0c4640 RCX: 0000000000000000 RDX: 0000000000000001 RSI: 0000000000000002 RDI: ffff88807ffdcc00 RBP: ffffea00014a5a00 R08: ffffffff846c1c01 R09: ffff88806d53b6d0 R10: ffff888006278000 R11: ffff8880062785bb R12: 0000000000000000 R13: 0000000000000001 R14: 0000000000000001 R15: 0000000000000001 __update_lru_size include/linux/mm_inline.h:48 [inline] update_lru_size include/linux/mm_inline.h:56 [inline] lruvec_add_folio include/linux/mm_inline.h:348 [inline] lru_add+0x44f/0x890 mm/swap.c:154 folio_batch_move_lru+0x110/0x4c0 mm/swap.c:172 __folio_batch_add_and_move+0x27e/0x7e0 mm/swap.c:196 shmem_alloc_and_add_folio mm/shmem.c:1991 [inline] shmem_get_folio_gfp.isra.0+0xc49/0x1410 mm/shmem.c:2556 shmem_get_folio mm/shmem.c:2662 [inline] shmem_write_begin+0x197/0x3b0 mm/shmem.c:3315 generic_perform_write+0x37f/0x800 mm/filemap.c:4314 shmem_file_write_iter+0x10d/0x140 mm/shmem.c:3490 new_sync_write fs/read_write.c:593 [inline] vfs_write fs/read_write.c:686 [inline] vfs_write+0xabc/0xe90 fs/read_write.c:666 ksys_write+0x121/0x240 fs/read_write.c:738 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xac/0x330 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x7f9b5abad69f Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 f9 92 02 00 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 31 44 89 c7 48 89 44 24 08 e8 4c 93 02 00 48 RSP: 002b:00007f9b595eddf0 EFLAGS: 00000293 ORIG_RAX: 0000000000000001 RAX: ffffffffffffffda RBX: 0000000000010000 RCX: 00007f9b5abad69f RDX: 0000000000010000 RSI: 00007f9b511ce000 RDI: 0000000000000007 RBP: 0000000000000000 R08: 0000000000000000 R09: 00000000000002f2 R10: 00000000000001ce R11: 0000000000000293 R12: 0000000000000007 R13: 00007f9b595edef0 R14: 00007f9b595edeb0 R15: 00007f9b511ce000