KCSAN: data-race in taskstats_exit / taskstats_exit

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    b4bd9343 x86, kcsan: Enable KCSAN for x86
git tree:       https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=125329db600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80
dashboard link: https://syzkaller.appspot.com/bug?extid=c5d03165a1bd1dead0c1
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+c5d03165a1bd1dead0c1@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KCSAN: data-race in taskstats_exit / taskstats_exit

write to 0xffff8881157bbe10 of 8 bytes by task 7951 on cpu 0:
 taskstats_tgid_alloc kernel/taskstats.c:567 [inline]
 taskstats_exit+0x6b7/0x717 kernel/taskstats.c:596
 do_exit+0x2c2/0x18e0 kernel/exit.c:864
 do_group_exit+0xb4/0x1c0 kernel/exit.c:983
 get_signal+0x2a2/0x1320 kernel/signal.c:2734
 do_signal+0x3b/0xc00 arch/x86/kernel/signal.c:815
 exit_to_usermode_loop+0x250/0x2c0 arch/x86/entry/common.c:159
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x2d7/0x2f0 arch/x86/entry/common.c:299
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881157bbe10 of 8 bytes by task 7949 on cpu 1:
 taskstats_tgid_alloc kernel/taskstats.c:559 [inline]
 taskstats_exit+0xb2/0x717 kernel/taskstats.c:596
 do_exit+0x2c2/0x18e0 kernel/exit.c:864
 do_group_exit+0xb4/0x1c0 kernel/exit.c:983
 __do_sys_exit_group kernel/exit.c:994 [inline]
 __se_sys_exit_group kernel/exit.c:992 [inline]
 __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:992
 do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7949 Comm: syz-executor.3 Not tainted 5.3.0+ #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: 7949 Comm: syz-executor.3 Not tainted 5.3.0+ #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+0x209/0x639 kernel/panic.c:219
 end_report kernel/kcsan/report.c:135 [inline]
 kcsan_report.cold+0x57/0xeb kernel/kcsan/report.c:283
 __kcsan_setup_watchpoint+0x342/0x500 kernel/kcsan/core.c:456
 __tsan_read8 kernel/kcsan/kcsan.c:31 [inline]
 __tsan_read8+0x2c/0x30 kernel/kcsan/kcsan.c:31
 taskstats_tgid_alloc kernel/taskstats.c:559 [inline]
 taskstats_exit+0xb2/0x717 kernel/taskstats.c:596
 do_exit+0x2c2/0x18e0 kernel/exit.c:864
 do_group_exit+0xb4/0x1c0 kernel/exit.c:983
 __do_sys_exit_group kernel/exit.c:994 [inline]
 __se_sys_exit_group kernel/exit.c:992 [inline]
 __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:992
 do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x459a59
Code: fd b7 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 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffc4ccf3408 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 000000000000001e RCX: 0000000000459a59
RDX: 0000000000413741 RSI: fffffffffffffff7 RDI: 0000000000000000
RBP: 0000000000000000 R08: 000000007f8e4506 R09: 00007ffc4ccf3460
R10: ffffffff81007108 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffc4ccf3460 R14: 0000000000000000 R15: 00007ffc4ccf3470
Kernel Offset: disabled
Rebooting in 86400 seconds..


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