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BUG: KCSAN: data-race in test_clear_page_writeback / writeout_period

write to 0xffffffff8418d3b8 of 8 bytes by interrupt on cpu 0:
 writeout_period+0xa6/0xe0 mm/page-writeback.c:623
 call_timer_fn+0x2e/0x1d0 kernel/time/timer.c:1417
 expire_timers+0x135/0x250 kernel/time/timer.c:1462
 __run_timers+0x358/0x420 kernel/time/timer.c:1731
 run_timer_softirq+0x2d/0x30 kernel/time/timer.c:1746
 __do_softirq+0x12c/0x26e kernel/softirq.c:558
 invoke_softirq kernel/softirq.c:432 [inline]
 __irq_exit_rcu+0x9a/0xb0 kernel/softirq.c:636
 sysvec_apic_timer_interrupt+0x69/0x80 arch/x86/kernel/apic/apic.c:1100
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:638
 dump_stack_lvl+0xec/0x103 arch/x86/include/asm/irqflags.h:45
 dump_stack+0x11/0x1a lib/dump_stack.c:112
 fail_dump lib/fault-inject.c:52 [inline]
 should_fail+0x23c/0x250 lib/fault-inject.c:146
 __should_failslab+0x81/0x90 mm/failslab.c:33
 should_failslab+0x5/0x20 mm/slab_common.c:1326
 slab_pre_alloc_hook mm/slab.h:494 [inline]
 slab_alloc_node mm/slab.c:3222 [inline]
 kmem_cache_alloc_node_trace+0x58/0x2e0 mm/slab.c:3617
 __do_kmalloc_node mm/slab.c:3639 [inline]
 __kmalloc_node_track_caller+0x30/0x40 mm/slab.c:3654
 kmalloc_reserve net/core/skbuff.c:355 [inline]
 __alloc_skb+0x187/0x420 net/core/skbuff.c:426
 alloc_skb include/linux/skbuff.h:1112 [inline]
 ip6_frag_next+0xaa/0x4b0 net/ipv6/ip6_output.c:772
 ip6_fragment+0x108b/0x1310 net/ipv6/ip6_output.c:978
 __ip6_finish_output net/ipv6/ip6_output.c:208 [inline]
 ip6_finish_output+0x444/0x480 net/ipv6/ip6_output.c:220
 NF_HOOK_COND include/linux/netfilter.h:296 [inline]
 ip6_output+0x10e/0x210 net/ipv6/ip6_output.c:243
 dst_output include/net/dst.h:448 [inline]
 ip6_local_out+0x121/0x1e0 net/ipv6/output_core.c:161
 ip6_send_skb net/ipv6/ip6_output.c:1934 [inline]
 ip6_push_pending_frames+0xb6/0x170 net/ipv6/ip6_output.c:1954
 rawv6_push_pending_frames net/ipv6/raw.c:613 [inline]
 rawv6_sendmsg+0x12ab/0x1380 net/ipv6/raw.c:956
 inet_sendmsg+0x5f/0x80 net/ipv4/af_inet.c:821
 sock_sendmsg_nosec net/socket.c:703 [inline]
 sock_sendmsg net/socket.c:723 [inline]
 __sys_sendto+0x2a8/0x370 net/socket.c:2019
 __do_sys_sendto net/socket.c:2031 [inline]
 __se_sys_sendto net/socket.c:2027 [inline]
 __x64_sys_sendto+0x74/0x90 net/socket.c:2027
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff8418d3b8 of 8 bytes by task 22 on cpu 1:
 wb_domain_writeout_inc mm/page-writeback.c:572 [inline]
 __wb_writeout_inc mm/page-writeback.c:593 [inline]
 test_clear_page_writeback+0x387/0x560 mm/page-writeback.c:2754
 end_page_writeback+0xd7/0x1b0 mm/filemap.c:1548
 ext4_finish_bio+0x3e2/0x430 fs/ext4/page-io.c:143
 ext4_release_io_end+0x98/0x200 fs/ext4/page-io.c:158
 ext4_end_io_end fs/ext4/page-io.c:192 [inline]
 ext4_do_flush_completed_IO fs/ext4/page-io.c:257 [inline]
 ext4_end_io_rsv_work+0x306/0x360 fs/ext4/page-io.c:271
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2276
 worker_thread+0x636/0xae0 kernel/workqueue.c:2422
 kthread+0x262/0x280 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

value changed: 0x00000000ffffb330 -> 0x00000000ffffb45c

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 22 Comm: kworker/u4:1 Not tainted 5.14.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: ext4-rsv-conversion ext4_end_io_rsv_work
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