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Date:   Sun, 22 Nov 2020 03:32:01 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Hillf Danton <hdanton@...a.com>,
        syzbot <syzbot+c0d52d0b3c0c3ffb9525@...kaller.appspotmail.com>
Cc:     axboe@...nel.dk, io-uring@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, syzkaller-bugs@...glegroups.com,
        viro@...iv.linux.org.uk, will@...nel.org
Subject: Re: INFO: task hung in __io_uring_files_cancel

On 22/11/2020 03:04, Hillf Danton wrote:
> On Sat, 21 Nov 2020 14:35:15 -0800
>> syzbot found the following issue on:
>>
>> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=10401726500000
>> final oops:     https://syzkaller.appspot.com/x/report.txt?x=12401726500000
>> console output: https://syzkaller.appspot.com/x/log.txt?x=14401726500000
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+c0d52d0b3c0c3ffb9525@...kaller.appspotmail.com
>> Fixes: 4d004099a668 ("lockdep: Fix lockdep recursion")
>>
>> INFO: task syz-executor.0:9557 blocked for more than 143 seconds.
>>       Not tainted 5.10.0-rc4-next-20201117-syzkaller #0
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> task:syz-executor.0  state:D stack:28584 pid: 9557 ppid:  8485 flags:0x00004002
>> Call Trace:
>>  context_switch kernel/sched/core.c:4269 [inline]
>>  __schedule+0x890/0x2030 kernel/sched/core.c:5019
>>  schedule+0xcf/0x270 kernel/sched/core.c:5098
>>  io_uring_cancel_files fs/io_uring.c:8720 [inline]
>>  io_uring_cancel_task_requests fs/io_uring.c:8772 [inline]
>>  __io_uring_files_cancel+0xc4d/0x14b0 fs/io_uring.c:8868
>>  io_uring_files_cancel include/linux/io_uring.h:51 [inline]
>>  exit_files+0xe4/0x170 fs/file.c:456
>>  do_exit+0xb61/0x29f0 kernel/exit.c:818
>>  do_group_exit+0x125/0x310 kernel/exit.c:920
>>  get_signal+0x3ea/0x1f70 kernel/signal.c:2750
>>  arch_do_signal_or_restart+0x2a6/0x1ea0 arch/x86/kernel/signal.c:811
>>  handle_signal_work kernel/entry/common.c:145 [inline]
>>  exit_to_user_mode_loop kernel/entry/common.c:169 [inline]
>>  exit_to_user_mode_prepare+0x124/0x200 kernel/entry/common.c:199
>>  syscall_exit_to_user_mode+0x38/0x260 kernel/entry/common.c:274
>>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> RIP: 0033:0x45deb9
>> Code: Unable to access opcode bytes at RIP 0x45de8f.
>> RSP: 002b:00007fa68397ccf8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
>> RAX: fffffffffffffe00 RBX: 000000000118bf28 RCX: 000000000045deb9
>> RDX: 0000000000000000 RSI: 0000000000000080 RDI: 000000000118bf28
>> RBP: 000000000118bf20 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118bf2c
>> R13: 00007fff50acc9af R14: 00007fa68397d9c0 R15: 000000000118bf2c
>>
...
> 
> Fix 311daef8013a ("io_uring: replace inflight_wait with tctx->wait")
> by cutting the pre-condition to wakeup because waitqueue_active()
> speaks the language in the East End while atomic_read() may speak
> the language in Paris.

Your description doesn't help, why do you think this is the problem?
->in_idle is always set when io_uring_cancel_files() sleeps on it,
and ->inflight_lock should guarantee ordering.

> 
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6082,8 +6082,7 @@ static void io_req_drop_files(struct io_
>  
>  	spin_lock_irqsave(&ctx->inflight_lock, flags);
>  	list_del(&req->inflight_entry);
> -	if (atomic_read(&tctx->in_idle))
> -		wake_up(&tctx->wait);
> +	wake_up(&tctx->wait);
>  	spin_unlock_irqrestore(&ctx->inflight_lock, flags);
>  	req->flags &= ~REQ_F_INFLIGHT;
>  	put_files_struct(req->work.identity->files);
> 

-- 
Pavel Begunkov

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