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BUG: KCSAN: data-race in lru_add_drain_all / lru_cache_add

write to 0xffff88813bd27550 of 1 bytes by task 28958 on cpu 1:
 pagevec_add include/linux/pagevec.h:74 [inline]
 lru_cache_add+0x7c/0x160 mm/swap.c:461
 shmem_getpage_gfp+0x8c4/0x1980 mm/shmem.c:1934
 shmem_getpage mm/shmem.c:155 [inline]
 shmem_file_read_iter+0x1b8/0x760 mm/shmem.c:2607
 call_read_iter include/linux/fs.h:1971 [inline]
 do_iter_readv_writev+0x2e8/0x360 fs/read_write.c:738
 do_iter_read+0x109/0x4b0 fs/read_write.c:801
 vfs_iter_read+0x4c/0x70 fs/read_write.c:843
 lo_read_simple drivers/block/loop.c:360 [inline]
 do_req_filebacked drivers/block/loop.c:636 [inline]
 loop_handle_cmd drivers/block/loop.c:2045 [inline]
 loop_queue_work+0xdfb/0x1330 drivers/block/loop.c:2063
 kthread_worker_fn+0x28f/0x540 kernel/kthread.c:735
 loop_kthread_worker_fn+0x3e/0x50 drivers/block/loop.c:932
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff88813bd27550 of 1 bytes by task 1638 on cpu 0:
 pagevec_count include/linux/pagevec.h:61 [inline]
 lru_add_drain_all+0x108/0x3c0 mm/swap.c:813
 khugepaged_do_scan+0x47/0x330 mm/khugepaged.c:2213
 khugepaged+0x75/0x680 mm/khugepaged.c:2274
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 1638 Comm: khugepaged Tainted: G        W         5.12.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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