Re: Using SQPOLL for-5.11/io_uring kernel NULL pointer dereference

[Pavel Begunkov <asml.silence@gmail.com>]

On 07/11/2020 16:28, Pavel Begunkov wrote:
> On 07/11/2020 14:09, Josef wrote:
>>> I haven't got the first email, is it "kernel NULL pointer dereference"
>>> as in the subject or just freeze?
>>
>> that's weird..probably the size of the attached log file is too big...
>> here dmesg log file
>> https://gist.github.com/1Jo1/3d0bcefc18f097265f0dc1ef054a87c0
> 
> That's much better with the log, thanks! I'll take a look later

Ok, we get into fget_many() without ->files, and it's clear how it
may happen. I'll write up a patch.

> 
>>
>>> - did you locate which test hangs it? If so what it uses? e.g. SQPOLL
>>> sharing, IOPOLL., etc.
>>
>> yes, it uses SQPOLL, without sharing, IPOLL is not enabled, and Async
>> Flag is enabled
>>
>>> - is it send/recvmsg, send/recv you use? any other?
>>
>> no the tests which occurs the error use these operations: OP_READ,
>> OP_WRITE, OP_POLL_ADD, OP_POLL_REMOVE, OP_CLOSE, OP_ACCEPT, OP_TIMEOUT
>> (OP_READ, OP_WRITE and OP_CLOSE async flag is enabled)
>>
>>> - does this happen often?
>>
>> yeah quite often
>>
>>> - you may try `funcgraph __io_sq_thread -H` or even with `io_sq_thread`
>>> (funcgraph is from bpftools). Or catch that with some other tools.
>>
>> I'm not quite familiar with these tools( kernel debugging in general)
>> I'll take a look tomorrow
> 

-- 
Pavel Begunkov