Waiman Long <longman@xxxxxxxxxx> writes: > On 4/4/20 1:55 AM, syzbot wrote: >> Hello, >> >> syzbot found the following crash on: >> >> HEAD commit: bef7b2a7 Merge tag 'devicetree-for-5.7' of git://git.kerne.. >> git tree: upstream >> console output: https://syzkaller.appspot.com/x/log.txt?x=15f39c5de00000 >> kernel config: https://syzkaller.appspot.com/x/.config?x=91b674b8f0368e69 >> dashboard link: https://syzkaller.appspot.com/bug?extid=a9fb1457d720a55d6dc5 >> compiler: gcc (GCC) 9.0.0 20181231 (experimental) >> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1454c3b7e00000 >> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12a22ac7e00000 >> >> The bug was bisected to: >> >> commit 7bc3e6e55acf065500a24621f3b313e7e5998acf >> Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> >> Date: Thu Feb 20 00:22:26 2020 +0000 >> >> proc: Use a list of inodes to flush from proc >> >> bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=165c4acde00000 >> final crash: https://syzkaller.appspot.com/x/report.txt?x=155c4acde00000 >> console output: https://syzkaller.appspot.com/x/log.txt?x=115c4acde00000 >> >> IMPORTANT: if you fix the bug, please add the following tag to the commit: >> Reported-by: syzbot+a9fb1457d720a55d6dc5@xxxxxxxxxxxxxxxxxxxxxxxxx >> Fixes: 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc") >> >> ======================================================== >> WARNING: possible irq lock inversion dependency detected >> 5.6.0-syzkaller #0 Not tainted >> -------------------------------------------------------- >> ksoftirqd/0/9 just changed the state of lock: >> ffffffff898090d8 (tasklist_lock){.+.?}-{2:2}, at: send_sigio+0xa9/0x340 fs/fcntl.c:800 >> but this lock took another, SOFTIRQ-unsafe lock in the past: >> (&pid->wait_pidfd){+.+.}-{2:2} >> >> >> and interrupts could create inverse lock ordering between them. >> >> >> other info that might help us debug this: >> Possible interrupt unsafe locking scenario: >> >> CPU0 CPU1 >> ---- ---- >> lock(&pid->wait_pidfd); >> local_irq_disable(); >> lock(tasklist_lock); >> lock(&pid->wait_pidfd); >> <Interrupt> >> lock(tasklist_lock); >> >> *** DEADLOCK *** > > That is a false positive. The qrwlock has the special property that it becomes > unfair (for read lock) at interrupt context. So unless it is taking a write lock > in the interrupt context, it won't go into deadlock. The current lockdep code > does not capture the full semantics of qrwlock leading to this false positive. > Whatever it was it was fixed with: 63f818f46af9 ("proc: Use a dedicated lock in struct pid") It is classic lock inversion caused by not disabling irqs. Unless I am completely mistaken any non-irq code path that does: write_lock_irq(&tasklist_lock); spin_lock(&pid->lock); Is susceptible to deadlock with: spin_lock(&pid->lock); <Interrupt> read_lock(&task_list_lock); Because it remains a lock inversion even with only a read lock taken in irq context in irq context. Eric
