Re: [syzbot] KCSAN: data-race in call_rcu / rcu_gp_fqs_loop

[Paul E. McKenney]

On Mon, Oct 25, 2021 at 12:31:53PM +0200, Marco Elver wrote:
> +Cc Paul
> 
> data race is in rcu code, presumably not yet discovered by rcutorture?

Quite possibly, and I will take a look.  Thank you for sending this
along.

							Thanx, Paul

> On Mon, 25 Oct 2021 at 12:29, syzbot
> <syzbot+4dfb96a94317a78f44d9@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    9c0c4d24ac00 Merge tag 'block-5.15-2021-10-22' of git://gi..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=159c4954b00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=6339b6ea86d89fd7
> > dashboard link: https://syzkaller.appspot.com/bug?extid=4dfb96a94317a78f44d9
> > compiler:       Debian clang version 11.0.1-2, GNU ld (GNU Binutils for Debian) 2.35.2
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+4dfb96a94317a78f44d9@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > ==================================================================
> > BUG: KCSAN: data-race in call_rcu / rcu_gp_fqs_loop
> >
> > write to 0xffffffff837342e0 of 8 bytes by task 11 on cpu 1:
> >  rcu_gp_fqs kernel/rcu/tree.c:1910 [inline]
> >  rcu_gp_fqs_loop+0x348/0x470 kernel/rcu/tree.c:1971
> >  rcu_gp_kthread+0x25/0x1a0 kernel/rcu/tree.c:2130
> >  kthread+0x262/0x280 kernel/kthread.c:319
> >  ret_from_fork+0x1f/0x30
> >
> > read to 0xffffffff837342e0 of 8 bytes by task 379 on cpu 0:
> >  __call_rcu_core kernel/rcu/tree.c:2904 [inline]
> >  __call_rcu kernel/rcu/tree.c:3020 [inline]
> >  call_rcu+0x4c0/0x6d0 kernel/rcu/tree.c:3067
> >  __dentry_kill+0x3ec/0x4e0 fs/dcache.c:596
> >  dput+0xc6/0x360 fs/dcache.c:888
> >  do_unlinkat+0x2a8/0x540 fs/namei.c:4172
> >  __do_sys_unlink fs/namei.c:4217 [inline]
> >  __se_sys_unlink fs/namei.c:4215 [inline]
> >  __x64_sys_unlink+0x2c/0x30 fs/namei.c:4215
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x44/0xa0 arch/x86/entry/common.c:80
> >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> >
> > value changed: 0x0000000000005c0d -> 0x0000000000005c0e
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 0 PID: 379 Comm: udevd Tainted: G        W         5.15.0-rc6-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > ==================================================================
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx.
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.