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BUG: KCSAN: data-race in del_timer / detach_if_pending

write to 0xffff8881218b89b8 of 8 bytes by task 9808 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:1246
 mod_delayed_work_on+0x50/0x100 kernel/workqueue.c:1703
 mod_delayed_work include/linux/workqueue.h:524 [inline]
 wb_queue_work+0x11a/0x290 fs/fs-writeback.c:188
 bdi_split_work_to_wbs+0x39d/0x6b0 fs/fs-writeback.c:876
 __writeback_inodes_sb_nr+0x192/0x1e0 fs/fs-writeback.c:2468
 try_to_writeback_inodes_sb+0x70/0x90 fs/fs-writeback.c:2517
 ext4_nonda_switch+0x12d/0x140 fs/ext4/inode.c:2897
 ext4_page_mkwrite+0x8c0/0xe50 fs/ext4/inode.c:5900
 do_page_mkwrite+0x100/0x210 mm/memory.c:2276
 wp_page_shared mm/memory.c:2603 [inline]
 do_wp_page+0xbf5/0xeb0 mm/memory.c:2715
 handle_pte_fault mm/memory.c:3961 [inline]
 __handle_mm_fault+0x1d16/0x2e00 mm/memory.c:4075
 handle_mm_fault+0x21b/0x530 mm/memory.c:4112
 do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
 __do_page_fault+0x456/0x8d0 arch/x86/mm/fault.c:1506
 do_page_fault+0x38/0x194 arch/x86/mm/fault.c:1530
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1203

read to 0xffff8881218b89b8 of 8 bytes by task 9700 on cpu 0:
 del_timer+0x3b/0xb0 kernel/time/timer.c:1198
 try_to_grab_pending+0x22d/0x3e0 kernel/workqueue.c:1246
 mod_delayed_work_on+0x50/0x100 kernel/workqueue.c:1703
 mod_delayed_work include/linux/workqueue.h:524 [inline]
 wb_queue_work+0x11a/0x290 fs/fs-writeback.c:188
 bdi_split_work_to_wbs+0x39d/0x6b0 fs/fs-writeback.c:876
 __writeback_inodes_sb_nr+0x192/0x1e0 fs/fs-writeback.c:2468
 try_to_writeback_inodes_sb+0x70/0x90 fs/fs-writeback.c:2517
 ext4_nonda_switch+0x12d/0x140 fs/ext4/inode.c:2897
 ext4_da_write_begin+0xc5/0x840 fs/ext4/inode.c:2938
 generic_perform_write+0x136/0x320 mm/filemap.c:3309
 ext4_buffered_write_iter+0x143/0x290 fs/ext4/file.c:252
 ext4_file_write_iter+0xf4/0xd40 fs/ext4/file.c:547
 call_write_iter include/linux/fs.h:1902 [inline]
 new_sync_write+0x388/0x4a0 fs/read_write.c:483
 __vfs_write+0xb1/0xc0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 9700 Comm: syz-executor.5 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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