Dne 13. 07. 20 v 12:08 Martin Wilck napsal(a):
Hi Zdenek,
On Mon, 2020-07-13 at 11:56 +0200, Zdenek Kabelac wrote:
Dne 13. 07. 20 v 11:21 Martin Wilck napsal(a):
Hi Lixiaokeng,
@Zdenek, do we have to protect every libdm call, or is it
sufficient
to protect only dm_task_run(), as lixiaokeng suggested?
Hi
It's actually hard to answer it in a simple way.
Several properties are held in library static variables. So
converting libdm
into a fully threaded 'version' would basically require to duplicate
all API
functions with extended 'context' structure passed in - where all
buffers can
be maintained properly (and it's getting more complicated with signal
handling
and debug logging).
Sure. I don't think this is where we're going for. But we need to
figure out how to handle this properly in libmultipath.
I take it that we must really protect *every* libdm call in
libmultipath and multipathd if we want to do it right.
ATM it doesn't look like there is a big need for threaded support of
DM usage
as majority of tools spends most of their time outside thus
'serialization'
of lvm2 or dmeventd on libdm access look doesn't look like a big
issue
(let's say there are far bigger fishes to hunt).
Agreed.
As for the issue of keeping control_fd open - there should be a
synchronized
call of dm_hold_control_dev(0/1) - see the codebase of dmeventd.c
in lvm2
code base - how we solve the control_fd handling.
Ben has already added support for dm_hold_control_dev() in libmultipath
(e24d8b1 ("libmutipath: don't close fd on dm_lib_release")). But this
doesn't protect us from calling _open_control() simultaneously in
separate code paths, as Lixiaokeng has pointed out.
I don't see how it would, as dm_hold_control_dev() also just changes
a static variable.
Here I'm not familiar with multipath codebase (more a question for Ben
likely), but is 'dm_hold...()' being used in non-threaded way - i.e. in
dmeventd these functions are called before/after threads are finished - so
there would be no way 2 threads may call this function in parallel ?
Zdenek
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