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BUG: KCSAN: data-race in page_cache_ra_unbounded / proc_cgroup_show

write to 0xffff88812df8d06c of 4 bytes by task 12268 on cpu 0:
 memalloc_nofs_save include/linux/sched/mm.h:249 [inline]
 page_cache_ra_unbounded+0xa8/0x410 mm/readahead.c:193
 ondemand_readahead+0x579/0x7b0 mm/readahead.c:267
 page_cache_sync_ra+0xaf/0xe0 mm/readahead.c:578
 page_cache_sync_readahead include/linux/pagemap.h:879 [inline]
 filemap_get_pages mm/filemap.c:2467 [inline]
 filemap_read+0x388/0x1230 mm/filemap.c:2550
 generic_file_read_iter+0x75/0x2c0 mm/filemap.c:2701
 ext4_file_read_iter+0x1db/0x290 fs/ext4/file.c:73
 call_read_iter include/linux/fs.h:2108 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x565/0x5d0 fs/read_write.c:496
 ksys_read+0xce/0x180 fs/read_write.c:634
 __do_sys_read fs/read_write.c:644 [inline]
 __se_sys_read fs/read_write.c:642 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:642
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88812df8d06c of 4 bytes by task 1030 on cpu 1:
 proc_cgroup_show+0x640/0x930 kernel/cgroup/cgroup.c:5829
 proc_single_show+0x84/0x100 fs/proc/base.c:774
 seq_read_iter+0x2ef/0x910 fs/seq_file.c:227
 seq_read+0x221/0x260 fs/seq_file.c:159
 vfs_read+0x154/0x5d0 fs/read_write.c:494
 ksys_read+0xce/0x180 fs/read_write.c:634
 __do_sys_read fs/read_write.c:644 [inline]
 __se_sys_read fs/read_write.c:642 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:642
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1030 Comm: systemd-journal Not tainted 5.13.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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