syzbot

==================================================================
BUG: KCSAN: data-race in ext4_remount / graft_tree

read to 0xffff888218543850 of 8 bytes by task 8431 on cpu 0:
 graft_tree+0x3c/0x160 fs/namespace.c:2172
 do_loopback fs/namespace.c:2307 [inline]
 do_mount+0x11fe/0x1560 fs/namespace.c:3137
 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 0xffff888218543850 of 8 bytes by task 8434 on cpu 1:
 ext4_remount+0x685/0x13a0 fs/ext4/super.c:5362
 legacy_reconfigure+0x9a/0xc0 fs/fs_context.c:670
 reconfigure_super+0x36a/0x570 fs/super.c:955
 do_remount fs/namespace.c:2551 [inline]
 do_mount+0x1188/0x1560 fs/namespace.c:3134
 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: 1 PID: 8434 Comm: syz-executor.5 Not tainted 5.4.0-rc6+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
==================================================================
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 8434 Comm: syz-executor.5 Not tainted 5.4.0-rc6+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0xf5/0x159 lib/dump_stack.c:113
 panic+0x210/0x640 kernel/panic.c:221
 kcsan_report.cold+0xc/0xe kernel/kcsan/report.c:302
 kcsan_setup_watchpoint+0x3fe/0x410 kernel/kcsan/core.c:357
 check_access kernel/kcsan/core.c:409 [inline]
 __tsan_unaligned_write8+0x143/0x1f0 kernel/kcsan/core.c:530
 ext4_remount+0x685/0x13a0 fs/ext4/super.c:5362
 legacy_reconfigure+0x9a/0xc0 fs/fs_context.c:670
 reconfigure_super+0x36a/0x570 fs/super.c:955
 do_remount fs/namespace.c:2551 [inline]
 do_mount+0x1188/0x1560 fs/namespace.c:3134
 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
RIP: 0033:0x45a219
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fdb6cd9cc78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 000000000045a219
RDX: 0000000000000000 RSI: 00000000200000c0 RDI: 0000000000000000
RBP: 000000000075bfc8 R08: 0000000000000000 R09: 0000000000000000
R10: 000000000302a162 R11: 0000000000000246 R12: 00007fdb6cd9d6d4
R13: 00000000004c6db2 R14: 00000000004dc528 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..