Re: [PATCH 6/7] ext4: endless truncate due to nonlocked dio readers V2

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Thu, Sep 13, 2012 at 02:07:36PM +0200, Jan Kara wrote:
>   Hello,
> 
> On Thu 13-09-12 18:41:36, Zheng Liu wrote:
> > Could you please provide more detailed workload to convince me?  I
> > am thinking about whether we really need to disable dioread_nolock
> > feature in here.  In our benchmarks, we don't see this problem.
>   I just did:
> 
> # Create file
> dd if=/dev/zero of=/mnt/file bs=1M count=30
> sync
> # Start 10 DIO dio readers in parallel reading the file in a loop
> for (( i = 0; i < 10; i++ )); do
>   while true; do
>     dd if=/mnt/file bs=4k iflag=direct of=/dev/null
>   done &
> done
> sleep 1
> 
> # Try to truncate the file - never finishes.
> truncate -s 16 /mnt/file
> 
> It is pretty easy to hit this. Besides being a DOS attack vector (but I
> won't be too concerned about this - there are plenty of ways how local
> process can screw you) I can easily imagine some application to get bitten
> by this. 

Hi Jan,

Thanks for your explanation, but in my desktop I cannot reproduce this
problem.  The size of `file' is 16.  Am I missing something?

Regards,
Zheng