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BUG: KCSAN: data-race in __rcu_read_unlock / rcu_preempt_deferred_qs_irqrestore

read to 0xffffffff85a7d1d8 of 8 bytes by task 2887 on cpu 1:
 rcu_read_unlock_special kernel/rcu/tree_plugin.h:614 [inline]
 __rcu_read_unlock+0x1f6/0x3c0 kernel/rcu/tree_plugin.h:383
 rcu_read_unlock include/linux/rcupdate.h:652 [inline]
 filemap_map_pages+0x5b3/0x990 mm/filemap.c:2688
 do_fault_around mm/memory.c:3450 [inline]
 do_read_fault mm/memory.c:3484 [inline]
 do_fault mm/memory.c:3618 [inline]
 handle_pte_fault mm/memory.c:3849 [inline]
 __handle_mm_fault+0x2544/0x2c70 mm/memory.c:3973
 handle_mm_fault+0x21b/0x530 mm/memory.c:4010
 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:1202

write to 0xffffffff85a7d1d8 of 8 bytes by task 6955 on cpu 0:
 rcu_preempt_deferred_qs_irqrestore+0x424/0x580 kernel/rcu/tree_plugin.h:498
 rcu_read_unlock_special kernel/rcu/tree_plugin.h:644 [inline]
 __rcu_read_unlock+0x16a/0x3c0 kernel/rcu/tree_plugin.h:383
 rcu_read_unlock include/linux/rcupdate.h:652 [inline]
 sock_def_readable+0xd5/0x140 net/core/sock.c:2801
 unix_dgram_sendmsg+0x92a/0xc80 net/unix/af_unix.c:1752
 sock_sendmsg_nosec net/socket.c:637 [inline]
 sock_sendmsg+0x9f/0xc0 net/socket.c:657
 __sys_sendto+0x21f/0x320 net/socket.c:1952
 __do_sys_sendto net/socket.c:1964 [inline]
 __se_sys_sendto net/socket.c:1960 [inline]
 __x64_sys_sendto+0x89/0xb0 net/socket.c:1960
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 6955 Comm: udevd 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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