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Re: [libvirt] [PATCH v7 2/6] add fuse support for libvirt lxc



On 11/07/2012 11:54 AM, Gao feng wrote:
> 于 2012年11月06日 20:55, Richard W.M. Jones 写道:
>> On Tue, Nov 06, 2012 at 07:50:33PM +0800, Gao feng wrote:
>>> 于 2012年11月06日 17:11, Richard W.M. Jones 写道:
>>>> On Tue, Nov 06, 2012 at 04:54:59PM +0800, Gao feng wrote:
>>>>> Because libvirt_lxc create a thread to mount fuse filesystem.
>>>>> and then libvirt_lxc will try to access the fuse filesystem.
>>>>>
>>>>> Without waiting,libvirt_lxc may access fuse before it is mounted.
>>>>>
>>>>> And I can't find a better way to make accessing fuse after the fuse
>>>>> being mounted,because fuse_main will return until fuse being unmounted,so
>>>>> I don't know how to pass message to tell libvirt_lxc whether the fuse is
>>>>> mounted success.
>>>>
>>>> Oh I see.  This is solvable, but I believe that you cannot use
>>>> fuse_main.  You have to look at the fuse sources (quite complex!)  and
>>>> do the individual fuse operations.
>>>>
>>>> Have a look at how libguestfs does it ...
>>>>
>>>>   https://github.com/libguestfs/libguestfs/blob/master/src/fuse.c#L911
>>>>   http://libguestfs.org/guestfs.3.html#mount-local
>>>>
>>>
>>>
>>> Thanks for the information.
>>>
>>> Can fuse_mount & fuse_new & fuse_loop avoid the waiting?
>>>
>>> It seems has the same problem.
>>
>> It waits, but it doesn't busy wait in a loop.  The first access to the
>> filesystem hangs until the thread which is handling the filesystem is
>> listening for requests.
>>
> 
> It makes me confuse.
> 
> There are two threads(A and B).
> thread A mounts fuse filesystem on mountpoint /mnt/fuse.
> thread B try to access file /mnt/fuse/meminfo. and we don't
> know which thread will execute first.
> 
> The information I got from your comment is that because thread A
> doesn't mount fuse to /mnt/fuse, when thread B access to the
> /mnt/fuse/meminfo,it will hang. I am right?

If I'm not wrong, I think Rich means that you should create the mountpoint
and the operations using fuse_mount and fuse_new before you create the thread B,
then make another thread A using fuse_loop to loop behind.  Then if thread B try
to access the mountpoint before thread A runs, B will hang until thread A runs into the loop.


Thanks,
Wanlong Gao

> 
> If I am right,I don't think this can be implemented by fuse.
> because the access of fuse file will make a fuse session message,
> if thread A doesn't call fuse_session_receive_buf to receive this message,
> The access of the fuse file will failed,not hang.
> 
> Maybe I misunderstand something.
> Can you explain this for me?
> 
> Thanks!
> 
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