linux-kernel - Re: [syzbot] INFO: task hung in sys_io

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Date:   Fri, 06 Aug 2021 16:41:14 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     syzbot <syzbot+d40a01556c761b2cb385@...kaller.appspotmail.com>
Cc:     bcrl@...ck.org, linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        viro@...iv.linux.org.uk
Subject: Re: [syzbot] INFO: task hung in sys_io_destroy

syzbot <syzbot+d40a01556c761b2cb385@...kaller.appspotmail.com> writes:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    1d67c8d993ba Merge tag 'soc-fixes-5.14-1' of git://git.ker..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11b40232300000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f1b998c1afc13578
> dashboard link: https://syzkaller.appspot.com/bug?extid=d40a01556c761b2cb385
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12453812300000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11225922300000
>
> Bisection is inconclusive: the issue happens on the oldest tested release.
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=127cac6a300000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=117cac6a300000
> console output: https://syzkaller.appspot.com/x/log.txt?x=167cac6a300000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d40a01556c761b2cb385@...kaller.appspotmail.com
>
> INFO: task syz-executor299:8807 blocked for more than 143 seconds.
>       Not tainted 5.14.0-rc1-syzkaller #0
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:syz-executor299 state:D stack:29400 pid: 8807 ppid:  8806 flags:0x00000000
> Call Trace:
>  context_switch kernel/sched/core.c:4683 [inline]
>  __schedule+0x93a/0x26f0 kernel/sched/core.c:5940
>  schedule+0xd3/0x270 kernel/sched/core.c:6019
>  schedule_timeout+0x1db/0x2a0 kernel/time/timer.c:1854
>  do_wait_for_common kernel/sched/completion.c:85 [inline]
>  __wait_for_common kernel/sched/completion.c:106 [inline]
>  wait_for_common kernel/sched/completion.c:117 [inline]
>  wait_for_completion+0x176/0x280 kernel/sched/completion.c:138
>  __do_sys_io_destroy fs/aio.c:1402 [inline]
>  __se_sys_io_destroy fs/aio.c:1380 [inline]
>  __x64_sys_io_destroy+0x17e/0x1e0 fs/aio.c:1380
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae

The reproducer is creating a thread, issuing a IOCB_CMD_PREAD from a
pipe in that thread, and then calling io_destroy from another thread.
Because there is no writer on the other end of the pipe, the read will
block.  Note that it also is not submitted asynchronously, as that's not
supported.

io_destroy is "hanging" because it's waiting for the read to finish.  If
the read thread is killed, cleanup happens as usual.  I'm not sure I
could classify this as a kernel bug.

-Jeff