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BUG: KCSAN: data-race in flush_workqueue / flush_workqueue

read to 0xffff888126afd050 of 8 bytes by task 11748 on cpu 1:
 flush_workqueue+0x30b/0xc00 kernel/workqueue.c:2834
 ceph_msgr_flush+0x2a/0x40 net/ceph/messenger.c:296
 ceph_mdsc_pre_umount+0x2a2/0x2e0 fs/ceph/mds_client.c:4245
 ceph_kill_sb+0x4f/0xd0 fs/ceph/super.c:1158
 deactivate_locked_super+0x64/0xb0 fs/super.c:335
 ceph_mount+0xbe1/0xd30 fs/ceph/super.c:1141
 legacy_get_tree+0x7e/0xf0 fs/fs_context.c:647
 vfs_get_tree+0x56/0x1a0 fs/super.c:1545
 do_new_mount fs/namespace.c:2823 [inline]
 do_mount+0x1049/0x1560 fs/namespace.c:3143
 ksys_mount+0xe8/0x160 fs/namespace.c:3352
 __do_sys_mount fs/namespace.c:3366 [inline]
 __se_sys_mount fs/namespace.c:3363 [inline]
 __x64_sys_mount+0x70/0x90 fs/namespace.c:3363
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff888126afd050 of 8 bytes by task 11710 on cpu 0:
 flush_workqueue+0x93f/0xc00 kernel/workqueue.c:2905
 ceph_msgr_flush+0x2a/0x40 net/ceph/messenger.c:296
 ceph_mdsc_destroy+0x3f/0xe0 fs/ceph/mds_client.c:4449
 destroy_fs_client+0x1f/0xa0 fs/ceph/super.c:743
 ceph_kill_sb+0xb7/0xd0 fs/ceph/super.c:1168
 deactivate_locked_super+0x64/0xb0 fs/super.c:335
 ceph_mount+0xbe1/0xd30 fs/ceph/super.c:1141
 legacy_get_tree+0x7e/0xf0 fs/fs_context.c:647
 vfs_get_tree+0x56/0x1a0 fs/super.c:1545
 do_new_mount fs/namespace.c:2823 [inline]
 do_mount+0x1049/0x1560 fs/namespace.c:3143
 ksys_mount+0xe8/0x160 fs/namespace.c:3352
 __do_sys_mount fs/namespace.c:3366 [inline]
 __se_sys_mount fs/namespace.c:3363 [inline]
 __x64_sys_mount+0x70/0x90 fs/namespace.c:3363
 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: 0 PID: 11710 Comm: syz-executor.0 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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