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BUG: KCSAN: data-race in writeback_sb_inodes / xas_set_mark

write to 0xffff88821afb4654 of 4 bytes by task 7868 on cpu 0:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:886 [inline]
 xas_set_mark+0x14d/0x160 lib/xarray.c:870
 __xa_set_mark+0xd3/0x100 lib/xarray.c:1706
 __set_page_dirty+0x152/0x270 fs/buffer.c:624
 mark_buffer_dirty+0x27f/0x2c0 fs/buffer.c:1152
 __ext4_handle_dirty_metadata+0x2b2/0x410 fs/ext4/ext4_jbd2.c:338
 ext4_do_update_inode fs/ext4/inode.c:5027 [inline]
 ext4_mark_iloc_dirty+0xdb3/0x1580 fs/ext4/inode.c:5578
 ext4_mark_inode_dirty+0xe9/0x420 fs/ext4/inode.c:5771
 ext4_dirty_inode+0xb1/0xe0 fs/ext4/inode.c:5802
 __mark_inode_dirty+0x5d4/0x940 fs/fs-writeback.c:2258
 mark_inode_dirty include/linux/fs.h:2183 [inline]
 generic_write_end+0x1b7/0x1f0 fs/buffer.c:2199
 ext4_da_write_end+0x158/0x620 fs/ext4/inode.c:3092
 generic_perform_write+0x1d3/0x320 mm/filemap.c:3298
 ext4_buffered_write_iter+0x143/0x290 fs/ext4/file.c:270
 ext4_file_write_iter+0xfa/0xd70 fs/ext4/file.c:642
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write+0x388/0x4a0 fs/read_write.c:483
 __vfs_write+0xb1/0xc0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88821afb4654 of 4 bytes by task 26413 on cpu 1:
 mapping_tagged include/linux/fs.h:521 [inline]
 requeue_inode fs/fs-writeback.c:1401 [inline]
 writeback_sb_inodes+0x6e3/0xa50 fs/fs-writeback.c:1743
 __writeback_inodes_wb+0xe8/0x1b0 fs/fs-writeback.c:1785
 wb_writeback+0x557/0x6a0 fs/fs-writeback.c:1894
 wb_do_writeback fs/fs-writeback.c:2037 [inline]
 wb_workfn+0x23e/0x9b0 fs/fs-writeback.c:2078
 process_one_work+0x413/0x8f0 kernel/workqueue.c:2264
 worker_thread+0xa0/0x800 kernel/workqueue.c:2410
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 26413 Comm: kworker/u4:9 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
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