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BUG: KCSAN: data-race in blk_mq_rq_ctx_init / blk_mq_rq_ctx_init

write to 0xffff888129969148 of 8 bytes by task 29087 on cpu 1:
 blk_mq_rq_ctx_init+0x4d4/0x5d0 block/blk-mq.c:346
 __blk_mq_alloc_request+0x2d2/0x2f0 block/blk-mq.c:402
 blk_mq_make_request+0x267/0x1010 block/blk-mq.c:2053
 do_make_request block/blk-core.c:1084 [inline]
 generic_make_request+0x240/0x5c0 block/blk-core.c:1156
 submit_bio+0x200/0x370 block/blk-core.c:1275
 submit_bh_wbc+0x34e/0x390 fs/buffer.c:3073
 submit_bh fs/buffer.c:3079 [inline]
 __sync_dirty_buffer+0xfa/0x170 fs/buffer.c:3165
 sync_dirty_buffer+0x16/0x20 fs/buffer.c:3178
 __ext4_handle_dirty_metadata+0x186/0x540 fs/ext4/ext4_jbd2.c:339
 ext4_splice_branch fs/ext4/indirect.c:463 [inline]
 ext4_ind_map_blocks+0x11e7/0x1660 fs/ext4/indirect.c:636
 ext4_map_blocks+0x6d3/0xf60 fs/ext4/inode.c:628
 ext4_iomap_alloc fs/ext4/inode.c:3402 [inline]
 ext4_iomap_begin+0x432/0x5c0 fs/ext4/inode.c:3441
 iomap_apply+0x8a/0x520 fs/iomap/apply.c:46
 iomap_dio_rw+0x501/0x8c0 fs/iomap/direct-io.c:501
 ext4_dio_write_iter fs/ext4/file.c:545 [inline]
 ext4_file_write_iter+0xd61/0xfd0 fs/ext4/file.c:653
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write fs/read_write.c:484 [inline]
 __vfs_write+0x2d8/0x340 fs/read_write.c:497
 vfs_write+0x19b/0x350 fs/read_write.c:559
 ksys_write+0xce/0x180 fs/read_write.c:612
 __do_sys_write fs/read_write.c:624 [inline]
 __se_sys_write fs/read_write.c:621 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:621
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888129969148 of 8 bytes by task 29106 on cpu 0:
 blk_mq_rq_ctx_init+0x4c2/0x5d0 block/blk-mq.c:346
 __blk_mq_alloc_request+0x2d2/0x2f0 block/blk-mq.c:402
 blk_mq_make_request+0x267/0x1010 block/blk-mq.c:2053
 do_make_request block/blk-core.c:1084 [inline]
 generic_make_request+0x240/0x5c0 block/blk-core.c:1156
 submit_bio+0x200/0x370 block/blk-core.c:1275
 submit_bh_wbc+0x34e/0x390 fs/buffer.c:3073
 submit_bh fs/buffer.c:3079 [inline]
 __sync_dirty_buffer+0xfa/0x170 fs/buffer.c:3165
 sync_dirty_buffer+0x16/0x20 fs/buffer.c:3178
 __ext4_handle_dirty_metadata+0x186/0x540 fs/ext4/ext4_jbd2.c:339
 ext4_splice_branch fs/ext4/indirect.c:463 [inline]
 ext4_ind_map_blocks+0x11e7/0x1660 fs/ext4/indirect.c:636
 ext4_map_blocks+0x6d3/0xf60 fs/ext4/inode.c:628
 ext4_iomap_alloc fs/ext4/inode.c:3402 [inline]
 ext4_iomap_begin+0x432/0x5c0 fs/ext4/inode.c:3441
 iomap_apply+0x8a/0x520 fs/iomap/apply.c:46
 iomap_dio_rw+0x501/0x8c0 fs/iomap/direct-io.c:501
 ext4_dio_write_iter fs/ext4/file.c:545 [inline]
 ext4_file_write_iter+0xd61/0xfd0 fs/ext4/file.c:653
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write fs/read_write.c:484 [inline]
 __vfs_write+0x2d8/0x340 fs/read_write.c:497
 vfs_write+0x19b/0x350 fs/read_write.c:559
 ksys_write+0xce/0x180 fs/read_write.c:612
 __do_sys_write fs/read_write.c:624 [inline]
 __se_sys_write fs/read_write.c:621 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:621
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 29106 Comm: syz-executor.2 Not tainted 5.8.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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