==================================================================
BUG: KCSAN: data-race in __bforget / sync_mapping_buffers

write to 0xffff8880a2e245a0 of 8 bytes by task 17063 on cpu 0:
 __remove_assoc_queue fs/buffer.c:481 [inline]
 fsync_buffers_list fs/buffer.c:752 [inline]
 sync_mapping_buffers+0x427/0x7d0 fs/buffer.c:548
 ext4_fsync_nojournal fs/ext4/fsync.c:86 [inline]
 ext4_sync_file+0x25e/0x6e0 fs/ext4/fsync.c:170
 vfs_fsync_range+0x107/0x120 fs/sync.c:200
 generic_write_sync include/linux/fs.h:2747 [inline]
 ext4_buffered_write_iter+0x369/0x3b0 fs/ext4/file.c:276
 ext4_file_write_iter+0x768/0x1060 include/linux/fs.h:784
 call_write_iter include/linux/fs.h:1882 [inline]
 do_iter_readv_writev+0x32e/0x3d0 fs/read_write.c:721
 do_iter_write+0x112/0x4b0 fs/read_write.c:1026
 vfs_iter_write+0x4c/0x70 fs/read_write.c:1067
 iter_file_splice_write+0x41a/0x770 fs/splice.c:750
 do_splice_from fs/splice.c:846 [inline]
 direct_splice_actor+0x95/0x160 fs/splice.c:1016
 splice_direct_to_actor+0x365/0x660 fs/splice.c:971
 do_splice_direct+0xf2/0x170 fs/splice.c:1059
 do_sendfile+0x56a/0xba0 fs/read_write.c:1548
 __do_sys_sendfile64 fs/read_write.c:1603 [inline]
 __se_sys_sendfile64 fs/read_write.c:1595 [inline]
 __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1595
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880a2e245a0 of 8 bytes by task 3629 on cpu 1:
 __bforget+0x22/0x120 fs/buffer.c:1188
 bforget include/linux/buffer_head.h:295 [inline]
 __ext4_forget+0x60/0x690 fs/ext4/ext4_jbd2.c:269
 ext4_free_blocks+0x364/0x18c0 fs/ext4/mballoc.c:5173
 ext4_ext_try_to_merge_up fs/ext4/extents.c:1829 [inline]
 ext4_ext_try_to_merge+0x2fa/0x310 fs/ext4/extents.c:1856
 ext4_convert_unwritten_extents_endio fs/ext4/extents.c:3720 [inline]
 ext4_ext_handle_unwritten_extents+0x625/0xaa0 fs/ext4/extents.c:3848
 ext4_ext_map_blocks+0x758/0x20c0 fs/ext4/extents.c:4118
 ext4_map_blocks+0x6bf/0xf40 fs/ext4/inode.c:625
 ext4_convert_unwritten_extents+0x16a/0x2d0 fs/ext4/extents.c:4749
 ext4_convert_unwritten_io_end_vec+0xa4/0x130 fs/ext4/extents.c:4788
 ext4_end_io_end fs/ext4/page-io.c:187 [inline]
 ext4_do_flush_completed_IO fs/ext4/page-io.c:260 [inline]
 ext4_end_io_rsv_work+0x198/0x360 fs/ext4/page-io.c:274
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 3629 Comm: kworker/u4:4 Not tainted 5.9.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: ext4-rsv-conversion ext4_end_io_rsv_work
==================================================================