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BUG: KCSAN: data-race in page_cache_ra_unbounded / proc_cgroup_show

write to 0xffff88801226102c of 4 bytes by task 24980 on cpu 1:
 memalloc_nofs_restore include/linux/sched/mm.h:259 [inline]
 page_cache_ra_unbounded+0x48c/0x4c0 mm/readahead.c:239
 do_page_cache_ra mm/readahead.c:267 [inline]
 force_page_cache_ra+0x226/0x260 mm/readahead.c:298
 page_cache_sync_ra+0x178/0x1a0 mm/readahead.c:572
 page_cache_sync_readahead include/linux/pagemap.h:840 [inline]
 filemap_get_pages+0x1ba/0x660 mm/filemap.c:2375
 filemap_read+0x1ba/0x7b0 mm/filemap.c:2458
 generic_file_read_iter+0x82/0x410 mm/filemap.c:2609
 blkdev_read_iter+0xb3/0xc0 fs/block_dev.c:1723
 call_read_iter include/linux/fs.h:1971 [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+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88801226102c of 4 bytes by task 4824 on cpu 0:
 proc_cgroup_show+0x3b3/0x6a0 kernel/cgroup/cgroup.c:5821
 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+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 4824 Comm: systemd-journal Tainted: G        W         5.12.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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