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BUG: KCSAN: data-race in do_notify_parent_cldstop / prepare_signal

write to 0xffff88812b1580e0 of 4 bytes by interrupt on cpu 0:
 prepare_signal+0xd21/0xdb0 kernel/signal.c:957
 send_sigqueue+0xea/0x450 kernel/signal.c:1860
 posix_timer_event kernel/time/posix-timers.c:354 [inline]
 posix_timer_fn+0xe8/0x210 kernel/time/posix-timers.c:380
 __run_hrtimer+0x154/0x4b0 kernel/time/hrtimer.c:1524
 __hrtimer_run_queues kernel/time/hrtimer.c:1588 [inline]
 hrtimer_interrupt+0x37c/0xa40 kernel/time/hrtimer.c:1650
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1080 [inline]
 __sysvec_apic_timer_interrupt+0xa3/0x280 arch/x86/kernel/apic/apic.c:1097
 asm_call_on_stack+0xf/0x20 arch/x86/entry/entry_64.S:706
 __run_on_irqstack arch/x86/include/asm/irq_stack.h:22 [inline]
 run_on_irqstack_cond arch/x86/include/asm/irq_stack.h:48 [inline]
 sysvec_apic_timer_interrupt+0x80/0xd0 arch/x86/kernel/apic/apic.c:1091
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:581
 arch_local_irq_restore arch/x86/include/asm/paravirt.h:770 [inline]
 kcsan_setup_watchpoint+0x44b/0x4a0 kernel/kcsan/core.c:563
 rcu_preempt_read_exit kernel/rcu/tree_plugin.h:359 [inline]
 __rcu_read_unlock+0x25/0x260 kernel/rcu/tree_plugin.h:392
 rcu_read_unlock include/linux/rcupdate.h:687 [inline]
 batadv_nc_process_nc_paths net/batman-adv/network-coding.c:698 [inline]
 batadv_nc_worker+0x57a/0xa00 net/batman-adv/network-coding.c:727
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff88812b1580e0 of 4 bytes by task 23480 on cpu 1:
 do_notify_parent_cldstop+0x1b2/0x280 kernel/signal.c:2057
 do_signal_stop+0x505/0x6d0 kernel/signal.c:2389
 get_signal+0x80d/0x1650 kernel/signal.c:2615
 arch_do_signal+0x25/0x270 arch/x86/kernel/signal.c:811
 exit_to_user_mode_loop kernel/entry/common.c:135 [inline]
 exit_to_user_mode_prepare+0x124/0x210 kernel/entry/common.c:166
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:241
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 23480 Comm: syz-executor.3 Not tainted 5.9.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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