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BUG: KCSAN: data-race in iov_iter_copy_from_user_atomic / transfer_xor

write to 0xffff8881454a6000 of 4096 bytes by task 30951 on cpu 0:
 instrument_copy_from_user include/linux/instrumented.h:136 [inline]
 copyin lib/iov_iter.c:200 [inline]
 iov_iter_copy_from_user_atomic+0x2e5/0xb60 lib/iov_iter.c:1053
 generic_perform_write+0x1e4/0x3a0 mm/filemap.c:3668
 __generic_file_write_iter+0x161/0x300 mm/filemap.c:3789
 blkdev_write_iter+0x1ea/0x2b0 fs/block_dev.c:1703
 call_write_iter include/linux/fs.h:2114 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x69d/0x770 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881454a6ada of 1 bytes by task 4503 on cpu 1:
 transfer_xor+0x15b/0x2e0 drivers/block/loop.c:114
 lo_do_transfer drivers/block/loop.c:267 [inline]
 lo_write_transfer drivers/block/loop.c:334 [inline]
 do_req_filebacked drivers/block/loop.c:626 [inline]
 loop_handle_cmd drivers/block/loop.c:2046 [inline]
 loop_queue_work+0x706/0x1310 drivers/block/loop.c:2064
 kthread_worker_fn+0x280/0x4d0 kernel/kthread.c:756
 loop_kthread_worker_fn+0x3e/0x50 drivers/block/loop.c:933
 kthread+0x1d0/0x1f0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

value changed: 0xc7 -> 0x00

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 4503 Comm: loop3 Tainted: G        W         5.13.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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BUG: KCSAN: data-race in iov_iter_copy_from_user_atomic / transfer_xor

write to 0xffff88812f103000 of 4096 bytes by task 31114 on cpu 0:
 instrument_copy_from_user include/linux/instrumented.h:136 [inline]
 copyin lib/iov_iter.c:200 [inline]
 iov_iter_copy_from_user_atomic+0x2e5/0xb60 lib/iov_iter.c:1053
 generic_perform_write+0x1e4/0x3a0 mm/filemap.c:3668
 __generic_file_write_iter+0x161/0x300 mm/filemap.c:3789
 blkdev_write_iter+0x1ea/0x2b0 fs/block_dev.c:1703
 call_write_iter include/linux/fs.h:2114 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x69d/0x770 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88812f1033cb of 1 bytes by task 4503 on cpu 1:
 transfer_xor+0x1ad/0x2e0 drivers/block/loop.c:114
 lo_do_transfer drivers/block/loop.c:267 [inline]
 lo_write_transfer drivers/block/loop.c:334 [inline]
 do_req_filebacked drivers/block/loop.c:626 [inline]
 loop_handle_cmd drivers/block/loop.c:2046 [inline]
 loop_queue_work+0x706/0x1310 drivers/block/loop.c:2064
 kthread_worker_fn+0x280/0x4d0 kernel/kthread.c:756
 loop_kthread_worker_fn+0x3e/0x50 drivers/block/loop.c:933
 kthread+0x1d0/0x1f0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

value changed: 0x00 -> 0x8b

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 4503 Comm: loop3 Tainted: G        W         5.13.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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