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BUG: KCSAN: data-race in relay_switch_subbuf / relay_switch_subbuf

write to 0xffff8880a798c638 of 8 bytes by task 2504 on cpu 0:
 relay_switch_subbuf+0x478/0x490 kernel/relay.c:790
 relay_reserve include/linux/relay.h:261 [inline]
 __blk_add_trace+0x801/0x840 kernel/trace/blktrace.c:272
 blk_add_trace_rq+0x15b/0x1a0 kernel/trace/blktrace.c:813
 blk_add_trace_rq_issue+0xec/0x100 kernel/trace/blktrace.c:827
 trace_block_rq_issue include/trace/events/block.h:207 [inline]
 blk_mq_start_request+0x21f/0x370 block/blk-mq.c:682
 scsi_mq_prep_fn drivers/scsi/scsi_lib.c:1602 [inline]
 scsi_queue_rq+0x943/0x1850 drivers/scsi/scsi_lib.c:1672
 blk_mq_dispatch_rq_list+0xbe9/0xe40 block/blk-mq.c:1273
 blk_mq_do_dispatch_sched+0x11d/0x260 block/blk-mq-sched.c:115
 blk_mq_sched_dispatch_requests+0x2b4/0x380 block/blk-mq-sched.c:211
 __blk_mq_run_hw_queue+0xb7/0x160 block/blk-mq.c:1403
 blk_mq_run_work_fn+0x57/0x70 block/blk-mq.c:1636
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
 worker_thread+0xa0/0x800 kernel/workqueue.c:2415
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

write to 0xffff8880a798c638 of 8 bytes by task 22104 on cpu 1:
 relay_switch_subbuf+0x478/0x490 kernel/relay.c:790
 relay_reserve include/linux/relay.h:261 [inline]
 __blk_add_trace+0x801/0x840 kernel/trace/blktrace.c:272
 blk_add_trace_bio.isra.0+0x12f/0x150 kernel/trace/blktrace.c:865
 blk_add_trace_bio_queue+0x3d/0x50 kernel/trace/blktrace.c:902
 trace_block_bio_queue include/trace/events/block.h:357 [inline]
 generic_make_request_checks+0xf34/0x1130 block/blk-core.c:966
 generic_make_request+0x3e/0x740 block/blk-core.c:1018
 submit_bio+0x96/0x3c0 block/blk-core.c:1190
 submit_bh_wbc+0x40f/0x460 fs/buffer.c:3095
 submit_bh fs/buffer.c:3101 [inline]
 __sync_dirty_buffer+0xd3/0x210 fs/buffer.c:3187
 sync_dirty_buffer+0x24/0x40 fs/buffer.c:3200
 __ext4_handle_dirty_metadata+0x19c/0x350 fs/ext4/ext4_jbd2.c:300
 ext4_handle_dirty_dirblock+0x20a/0x280 fs/ext4/namei.c:404
 add_dirent_to_buf+0x271/0x390 fs/ext4/namei.c:2026
 ext4_add_entry+0x37b/0x8f0 fs/ext4/namei.c:2225
 ext4_add_nondir+0x31/0xa0 fs/ext4/namei.c:2553
 ext4_symlink+0x7c0/0x970 fs/ext4/namei.c:3331
 vfs_symlink fs/namei.c:4119 [inline]
 vfs_symlink+0x218/0x310 fs/namei.c:4105
 do_symlinkat+0x1a5/0x1e0 fs/namei.c:4146
 __do_sys_symlink fs/namei.c:4165 [inline]
 __se_sys_symlink fs/namei.c:4163 [inline]
 __x64_sys_symlink+0x3f/0x50 fs/namei.c:4163
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 22104 Comm: syz-executor.4 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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