[linux-lvm] lvm_t handles and thread-safety

[linux-lvm] lvm_t handles and thread-safety

[Alexander Lyakas]

Hello everybody,
I am using stock ubuntu natty with lvmlib 2.02.66.

In my application I would like to have several threads, each operating
on a different VG.

When I tried to share the same lvm_t handle between the threads, very
soon I got segmentation faults in dm_hash operations. Looking at the
code, I indeed saw that the contents of lvm_t are used without
synchronization.

So I tried to use a lvm_t handle per thread. Quite soon I ran into the
following scenario:
- when a thread calls lvm_init(), this function calls init_locking().
By default LVM uses file-based locking. So init_file_locking() is
called. This function resets the global _lock_list structure. So any
previous locks are forgotten.
- as a result, when another thread wants to unlock a previously-locked
VG, it scans the _lock_list, does not find the lock there and returns
an error (although lvm_vg_close() hides that error by always returning
0).
- however, the dm_hash that contains the locked VG name is not updated
and still contains that name. So the user thinks that the VG got
unlocked, but dm_hash thinks otherwise. Since now there is no way to
unlock this VG (it�s file-lock was forgotten), the dm_hash entry
cannot be removed. This might prevent locking other VGs (because of VG
lock order enforcement), but from the user�s point of view, he should
be able to lock new VGs.

Can somebody of the lvmlib developers please comment on those issues?
Basically, as I see it, my application should use a single lvm_t
handle and do not share access to it, or synchronize all access
between lvm_vg_create/lvm_vg_open and lvm_vg_close.

Are there any plans or intentions to provide some support for
threading with lvmlib?

Thanks,
  Alex.

Re: [linux-lvm] lvm_t handles and thread-safety

[Zdenek Kabelac]

Dne 1.8.2011 10:12, Alexander Lyakas napsal(a):
> Hello everybody,
> I am using stock ubuntu natty with lvmlib 2.02.66.
> 
> In my application I would like to have several threads, each operating
> on a different VG.
> 
> When I tried to share the same lvm_t handle between the threads, very
> soon I got segmentation faults in dm_hash operations. Looking at the
> code, I indeed saw that the contents of lvm_t are used without
> synchronization.


lvm library is not thread safe and it's quite far from being thread safe.

So there is no point on having 2 handles on 2 VGs - it will not work.

> 
> Are there any plans or intentions to provide some support for
> threading with lvmlib?
> 

With current version of lvm2 code there is no way.

Just use the pthread mutex around the lvm command. It's serialized in kernel
anyway. So you cannot gain any speed by using threads. (Check the clvmd code.)


Zdenek

Re: [linux-lvm] lvm_t handles and thread-safety

[Alexander Lyakas]

Zdenek,
thank you for the clarification.

Alex.


On Mon, Aug 1, 2011 at 2:05 PM, Zdenek Kabelac <zdenek.kabelac@gmail.com> wrote:
> Dne 1.8.2011 10:12, Alexander Lyakas napsal(a):
>> Hello everybody,
>> I am using stock ubuntu natty with lvmlib 2.02.66.
>>
>> In my application I would like to have several threads, each operating
>> on a different VG.
>>
>> When I tried to share the same lvm_t handle between the threads, very
>> soon I got segmentation faults in dm_hash operations. Looking at the
>> code, I indeed saw that the contents of lvm_t are used without
>> synchronization.
>
>
> lvm library is not thread safe and it's quite far from being thread safe.
>
> So there is no point on having 2 handles on 2 VGs - it will not work.
>
>>
>> Are there any plans or intentions to provide some support for
>> threading with lvmlib?
>>
>
> With current version of lvm2 code there is no way.
>
> Just use the pthread mutex around the lvm command. It's serialized in kernel
> anyway. So you cannot gain any speed by using threads. (Check the clvmd code.)
>
>
> Zdenek
>
>
>