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BUG: KCSAN: data-race in del_timer / detach_if_pending

write to 0xffff8880b4c91db8 of 8 bytes by task 24135 on cpu 1:
 __hlist_del include/linux/list.h:764 [inline]
 detach_timer kernel/time/timer.c:815 [inline]
 detach_if_pending+0xcd/0x290 kernel/time/timer.c:832
 del_timer kernel/time/timer.c:1200 [inline]
 del_timer+0x67/0xb0 kernel/time/timer.c:1190
 try_to_grab_pending+0x22d/0x3e0 kernel/workqueue.c:1251
 mod_delayed_work_on+0x50/0x100 kernel/workqueue.c:1708
 mod_delayed_work include/linux/workqueue.h:524 [inline]
 wb_wakeup+0x89/0x90 fs/fs-writeback.c:156
 wb_start_writeback fs/fs-writeback.c:1095 [inline]
 __wakeup_flusher_threads_bdi fs/fs-writeback.c:2113 [inline]
 __wakeup_flusher_threads_bdi+0x152/0x190 fs/fs-writeback.c:2104
 wakeup_flusher_threads+0xe4/0x170 fs/fs-writeback.c:2139
 shrink_inactive_list+0x8bb/0x920 mm/vmscan.c:2027
 shrink_list mm/vmscan.c:2285 [inline]
 shrink_node_memcg+0x55c/0xf80 mm/vmscan.c:2603
 shrink_node+0x164/0xb30 mm/vmscan.c:2808
 shrink_zones mm/vmscan.c:3025 [inline]
 do_try_to_free_pages+0x242/0x940 mm/vmscan.c:3083
 try_to_free_mem_cgroup_pages+0x1fd/0x4c0 mm/vmscan.c:3383
 reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349
 mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430
 tracehook_notify_resume include/linux/tracehook.h:197 [inline]
 exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x353/0x370 arch/x86/entry/common.c:300
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880b4c91db8 of 8 bytes by task 32666 on cpu 0:
 del_timer+0x3b/0xb0 kernel/time/timer.c:1198
 try_to_grab_pending+0x22d/0x3e0 kernel/workqueue.c:1251
 mod_delayed_work_on+0x50/0x100 kernel/workqueue.c:1708
 mod_delayed_work include/linux/workqueue.h:524 [inline]
 wb_wakeup+0x89/0x90 fs/fs-writeback.c:156
 wb_start_writeback fs/fs-writeback.c:1095 [inline]
 __wakeup_flusher_threads_bdi fs/fs-writeback.c:2113 [inline]
 __wakeup_flusher_threads_bdi+0x152/0x190 fs/fs-writeback.c:2104
 wakeup_flusher_threads+0xe4/0x170 fs/fs-writeback.c:2139
 shrink_inactive_list+0x8bb/0x920 mm/vmscan.c:2027
 shrink_list mm/vmscan.c:2285 [inline]
 shrink_node_memcg+0x55c/0xf80 mm/vmscan.c:2603
 shrink_node+0x164/0xb30 mm/vmscan.c:2808
 shrink_zones mm/vmscan.c:3025 [inline]
 do_try_to_free_pages+0x242/0x940 mm/vmscan.c:3083
 try_to_free_mem_cgroup_pages+0x1fd/0x4c0 mm/vmscan.c:3383
 reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349
 mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430
 tracehook_notify_resume include/linux/tracehook.h:197 [inline]
 exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath+0x20f/0x220 arch/x86/entry/common.c:274
 ret_from_fork+0x15/0x30 arch/x86/entry/entry_64.S:344

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 32666 Comm: syz-executor.3 Not tainted 5.4.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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