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BUG: KCSAN: data-race in relay_switch_subbuf / relay_switch_subbuf

write to 0xffff888051bfb438 of 8 bytes by task 31253 on cpu 1:
 relay_switch_subbuf+0x336/0x340 kernel/relay.c:796
 relay_reserve include/linux/relay.h:261 [inline]
 __blk_add_trace+0x6db/0x7a0 kernel/trace/blktrace.c:274
 blk_add_trace_bio_remap+0x1d1/0x1e0 kernel/trace/blktrace.c:1070
 trace_block_bio_remap include/trace/events/block.h:573 [inline]
 blk_partition_remap+0x456/0x4a0 block/blk-core.c:761
 submit_bio_checks+0x1f9/0xb20 block/blk-core.c:829
 submit_bio_noacct+0x33/0x930 block/blk-core.c:1044
 submit_bio+0x1f3/0x360 block/blk-core.c:1129
 ext4_io_submit+0xcd/0xf0 fs/ext4/page-io.c:382
 ext4_writepages+0x68c/0x1e10 fs/ext4/inode.c:2748
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __filemap_fdatawrite_range+0x19d/0x1d0 mm/filemap.c:422
 file_write_and_wait_range+0x9f/0x120 mm/filemap.c:761
 ext4_sync_file+0xf4/0x6a0 fs/ext4/fsync.c:151
 vfs_fsync_range+0x107/0x120 fs/sync.c:200
 generic_write_sync include/linux/fs.h:2723 [inline]
 ext4_buffered_write_iter+0x384/0x3d0 fs/ext4/file.c:278
 ext4_file_write_iter+0x45e/0x1090 fs/ext4/file.c:503
 call_write_iter include/linux/fs.h:1887 [inline]
 do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
 do_iter_write+0x112/0x4b0 fs/read_write.c:866
 vfs_iter_write+0x4c/0x70 fs/read_write.c:907
 iter_file_splice_write+0x42a/0x780 fs/splice.c:686
 do_splice_from fs/splice.c:764 [inline]
 direct_splice_actor+0x80/0xa0 fs/splice.c:933
 splice_direct_to_actor+0x345/0x650 fs/splice.c:888
 do_splice_direct+0xf5/0x170 fs/splice.c:976
 do_sendfile+0x574/0xb70 fs/read_write.c:1257
 __do_sys_sendfile64 fs/read_write.c:1318 [inline]
 __se_sys_sendfile64 fs/read_write.c:1304 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1304
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff888051bfb438 of 8 bytes by task 31246 on cpu 0:
 relay_switch_subbuf+0x336/0x340 kernel/relay.c:796
 relay_reserve include/linux/relay.h:261 [inline]
 __blk_add_trace+0x6db/0x7a0 kernel/trace/blktrace.c:274
 blk_add_trace_bio kernel/trace/blktrace.c:908 [inline]
 blk_add_trace_bio_queue+0x133/0x140 kernel/trace/blktrace.c:946
 trace_block_bio_queue include/trace/events/block.h:371 [inline]
 submit_bio_checks+0xadf/0xb20 block/blk-core.c:909
 submit_bio_noacct+0x33/0x930 block/blk-core.c:1044
 submit_bio+0x1f3/0x360 block/blk-core.c:1129
 ext4_io_submit+0xcd/0xf0 fs/ext4/page-io.c:382
 ext4_writepages+0x68c/0x1e10 fs/ext4/inode.c:2748
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __filemap_fdatawrite_range+0x19d/0x1d0 mm/filemap.c:422
 file_write_and_wait_range+0x9f/0x120 mm/filemap.c:761
 ext4_sync_file+0xf4/0x6a0 fs/ext4/fsync.c:151
 vfs_fsync_range+0x107/0x120 fs/sync.c:200
 generic_write_sync include/linux/fs.h:2723 [inline]
 ext4_buffered_write_iter+0x384/0x3d0 fs/ext4/file.c:278
 ext4_file_write_iter+0x45e/0x1090 fs/ext4/file.c:503
 call_write_iter include/linux/fs.h:1887 [inline]
 do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
 do_iter_write+0x112/0x4b0 fs/read_write.c:866
 vfs_iter_write+0x4c/0x70 fs/read_write.c:907
 iter_file_splice_write+0x42a/0x780 fs/splice.c:686
 do_splice_from fs/splice.c:764 [inline]
 direct_splice_actor+0x80/0xa0 fs/splice.c:933
 splice_direct_to_actor+0x345/0x650 fs/splice.c:888
 do_splice_direct+0xf5/0x170 fs/splice.c:976
 do_sendfile+0x574/0xb70 fs/read_write.c:1257
 __do_sys_sendfile64 fs/read_write.c:1318 [inline]
 __se_sys_sendfile64 fs/read_write.c:1304 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1304
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 31246 Comm: syz-executor.4 Not tainted 5.10.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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