On Tue, May 29, 2018 at 01:40:34PM +0800, Anand Jain wrote:
> >>> v3->v4: As we traverse through the seed device, fs_device gets
> >>> updated with
> >>> the child seed fs_devices, so make sure we use the same fs_devices
> >>> pointer for the mutex_unlock as used for the mutex_lock.
> >>
> >> Well, now that I see the change, shouldn't we always hold the
> >> device_list_mutex of the fs_devices that's being processed? Ie. each
> >> time it's switched, the previous is unlocked and new one locked.
> >
> > No David. That's because we organize seed device under its parent
> > fs_devices ((fs_devices::seed)::seed)..so on, and they are a local
> > cloned copy of the original seed fs_devices. So parent's
> > fs_devices::device_list_mutex lock will suffice.
>
> On the 2nd thought, though theoretically you are right. But practically
> there isn't any use case which can benefit by using the intricate
> locking as you suggested above.
I don't think it's intricate or complex, just lock the fs_devices that
can be potentially modified in the following loop.
Schematically:
function called with some fs_devices
again:
lock fs_devices->device_list_mutex
foreach device in fs_devices
if ...
fs_devices counters get changed after device
deletion
endfor
if (fs_devices->seed)
unlock fs_devices->device_list_mutex
fs_devices = fs_devices->seed
lock fs_devices->device_list_mutex <-- lock the new one
goto again
endif
unlock fs_devices->device_list_mutex <-- correctly unlock
endfunc
> I am following the following method:-
> By holding the parent fs_devices (which is also the mounted fs_devices
> lock) it would imply to lock its dependent cloned seed fs_devices, as
> to reach the cloned seed device, first we need to traverse from the
> parent fs_devices.
Locking the parent fs_devices might be the right scheme, but if any
child fs_devices pointer gets passed to a random function that will
change it, then the locking will not protect it.
This might need a deeper audit how the seeding device is done and maybe
add some helpers that eg. lock the whole chain so all of them are
protected.
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