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BUG: KCSAN: data-race in __dentry_kill / vfs_unlink

read-write to 0xffff8880618a9a80 of 4 bytes by task 8451 on cpu 1:
 dont_mount include/linux/dcache.h:355 [inline]
 vfs_unlink+0x269/0x3b0 fs/namei.c:3837
 do_unlinkat+0x28a/0x4d0 fs/namei.c:3899
 __do_sys_unlink fs/namei.c:3945 [inline]
 __se_sys_unlink fs/namei.c:3943 [inline]
 __x64_sys_unlink+0x2c/0x30 fs/namei.c:3943
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880618a9a80 of 4 bytes by task 31135 on cpu 0:
 dentry_unlist fs/dcache.c:544 [inline]
 __dentry_kill+0x1e7/0x450 fs/dcache.c:575
 shrink_dentry_list+0x4c8/0x590 fs/dcache.c:1148
 prune_dcache_sb+0x72/0xa0 fs/dcache.c:1229
 super_cache_scan+0x23e/0x2e0 fs/super.c:105
 do_shrink_slab+0x28b/0x550 mm/vmscan.c:511
 shrink_slab+0xea/0x1c0 mm/vmscan.c:672
 drop_slab_node mm/vmscan.c:706 [inline]
 drop_slab+0xe5/0x160 mm/vmscan.c:716
 drop_caches_sysctl_handler+0xd0/0x1a0 fs/drop_caches.c:65
 proc_sys_call_handler+0x329/0x4e0 fs/proc/proc_sysctl.c:591
 proc_sys_write+0x1e/0x30 fs/proc/proc_sysctl.c:617
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6d4/0x7c0 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 31135 Comm: syz-executor.5 Not tainted 5.11.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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