[Ocfs2-devel] [PATCH] ocfs2: don't use iocb when EIOCBQUEUED returns

[Changwei Ge]

Hi Jun,

On 2018/4/11 9:52, piaojun wrote:
> Hi Changwei,
> 
> It seems other codes which try to access 'iocb' will also cause error,
> right? I think we should find the reason why 'iocb' is freed first.

Which code snippet do you mean? Actually, I have checked most of other parts in 
write_iter() and read_iter(). I don't see any risk accessing freed iocb yet.

The root cause is clear for this issue.
iocb is freed by aio_complete().

You can refer to path:

dio_complete
   aio_complete
     kiocb_free

Thanks,
Changwei

> 
> thanks,
> Jun
> 
> On 2018/4/11 9:07, Changwei Ge wrote:
>> Hi Jun,
>>
>> On 2018/4/11 8:52, piaojun wrote:
>>> Hi Changwei,
>>>
>>> It looks like a code bug, and 'iocb' should not be freed at this place.
>>> Could this BUG reproduced easily?
>>
>> Actually, it's not easy to be reproduced since IO is much slower than CPU
>> executing instructions. But the logic here is broken, we'd better fix this.
>>
>> Thanks,
>> Changwei
>>
>>>
>>> thanks,
>>> Jun
>>>
>>> On 2018/4/10 20:00, Changwei Ge wrote:
>>>> When -EIOCBQUEUED returns, it means that aio_complete() will be called
>>>> from dio_complete(), which is an asynchronous progress against write_iter.
>>>> Generally, IO is a very slow progress than executing instruction, but we
>>>> still can't take the risk to access a freed iocb.
>>>>
>>>> And we do face a BUG crash issue.
>>>> >From crash tool, iocb is obviously freed already.
>>>> crash> struct -x kiocb ffff881a350f5900
>>>> struct kiocb {
>>>>     ki_filp = 0xffff881a350f5a80,
>>>>     ki_pos = 0x0,
>>>>     ki_complete = 0x0,
>>>>     private = 0x0,
>>>>     ki_flags = 0x0
>>>> }
>>>>
>>>> And the backtrace shows:
>>>> ocfs2_file_write_iter+0xcaa/0xd00 [ocfs2]
>>>> ? ocfs2_check_range_for_refcount+0x150/0x150 [ocfs2]
>>>> aio_run_iocb+0x229/0x2f0
>>>> ? try_to_wake_up+0x380/0x380
>>>> do_io_submit+0x291/0x540
>>>> ? syscall_trace_leave+0xad/0x130
>>>> SyS_io_submit+0x10/0x20
>>>> system_call_fastpath+0x16/0x75
>>>>
>>>> Signed-off-by: Changwei Ge <ge.changwei at h3c.com>
>>>> ---
>>>>    fs/ocfs2/file.c | 4 ++--
>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
>>>> index 5d1784a..1393ff2 100644
>>>> --- a/fs/ocfs2/file.c
>>>> +++ b/fs/ocfs2/file.c
>>>> @@ -2343,7 +2343,7 @@ static ssize_t ocfs2_file_write_iter(struct kiocb *iocb,
>>>>    
>>>>    	written = __generic_file_write_iter(iocb, from);
>>>>    	/* buffered aio wouldn't have proper lock coverage today */
>>>> -	BUG_ON(written == -EIOCBQUEUED && !(iocb->ki_flags & IOCB_DIRECT));
>>>> +	BUG_ON(written == -EIOCBQUEUED && !direct_io);
>>>>    
>>>>    	/*
>>>>    	 * deep in g_f_a_w_n()->ocfs2_direct_IO we pass in a ocfs2_dio_end_io
>>>> @@ -2463,7 +2463,7 @@ static ssize_t ocfs2_file_read_iter(struct kiocb *iocb,
>>>>    	trace_generic_file_aio_read_ret(ret);
>>>>    
>>>>    	/* buffered aio wouldn't have proper lock coverage today */
>>>> -	BUG_ON(ret == -EIOCBQUEUED && !(iocb->ki_flags & IOCB_DIRECT));
>>>> +	BUG_ON(ret == -EIOCBQUEUED && !direct_io);
>>>>    
>>>>    	/* see ocfs2_file_write_iter */
>>>>    	if (ret == -EIOCBQUEUED || !ocfs2_iocb_is_rw_locked(iocb)) {
>>>>
>>>
>> .
>>
>