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

[Changwei Ge]

Hi Gang,

I don't think this patch is a workaround trick.

We do face the risk using freed iocb although it is actually indeed hard 
to encounter, it still exists.

So I propose to fix it making ocfs2 more reliable.


Moreover, this patch has been kept in -mm tree for one month. Can anyone 
help review it with ack or nack? So I can do some improvement for it. :-)


Thanks,

Changwei


On 04/11/2018 10:51 AM, Gang He wrote:
> Hi Changwei,
>
> The code change just works around the problem, but theoretically the IOCB object should not be freed before which is handled.
> Anyway, if we can find the root cause behind via some way (e.g. inject delay in some place), the result is more perfect.
>
>
> Thanks
> Gang
>
>
>> 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)) {
>>>>
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