[libvirt] [PATCH] LXC: make sure fuse thread start to run before we do clone

[Gao feng]

On 11/13/2013 10:53 PM, Daniel P. Berrange wrote:
> On Fri, Nov 08, 2013 at 02:42:26PM -0500, Rich Felker wrote:
>> On Fri, Nov 08, 2013 at 01:30:09PM +0800, Daniel P. Berrange wrote:
>>> On Thu, Nov 07, 2013 at 09:15:43PM +0800, Gao feng wrote:
>>>> I met a problem that container blocked by seteuid/setegid
>>>> which is call in lxcContainerSetID on UP system and libvirt
>>>> compiled with --with-fuse=yes.
>>>>
>>>> I looked into the glibc's codes, and found setxid in glibc
>>>> calls futex() to wait for other threads to change their
>>>> setxid_futex to 0(see setxid_mark_thread in glibc).
>>>>
>>>> since the process created by clone system call will not
>>>> share the memory with the other threads and the context
>>>> of memory doesn't changed until we call execl.(COW)
>>>>
>>>> So if the process which created by clone is called before
>>>> fuse thread being stated, the new setxid_futex of fuse
>>>> thread will not be saw in this process, it will be blocked
>>>> forever.
>>>>
>>>> Maybe this problem should be fixed in glibc, but I send
>>>> this patch as a quick fix.
>>>
>>> Can you show a stack trace of the threads/processes deadlocking
>>
>> I think this is a symptom of setxid not being async-signal-safe like
>> it's required to be. I'm not sure if we have a bug tracker entry for
>> that; if not, it should be added. But if clone() is being used except
>> in a fork-like manner, this is probably invalid application usage too.
> 
> We are not using clone() in a manner that is strictly equivalent
> to fork(). Libvirt is using clone() to create Linux containers
> with new namespaces. eg we do 
> 
>   clone(CLONE_NEWPID|CLONE_NEWNS|CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWUSER|CLONE_NEWNET|SIGCHLD)
> 
> 
> IIUC, if a process is multi-threaded you should restrict yourself to
> use of async signal safe functions in between fork() and exec(). I
> assume this restriction applies to clone() and exec() pairings too.
> 
> Libvirt is in fact violating rules about only using async signal safe
> functions between clone() and exec() in many places. So I think what
> we need to do is avoid starting any threads in the parent until after
> we've clone()'d to create the new child namespace.

Thanks for fuse, any tring to access files exported by fuse will be blocked until
the fuse thread starts do fuse_loop.

I will post a update.

Thanks guys.