Hello, Linus.
On Tue, Jan 15, 2013 at 09:36:57AM -0800, Linus Torvalds wrote:
> Tejun, comments? You can see the whole thread on lkml, but the basic
> problem is that the module loading doing the unconditional
> async_synchronize_full() has caused problems, because we have
>
> - load module A
> - module A does per-controller async discovery of its devices (eg
> scsi or ata probing)
> - in the async thread, it initializes somethign that needs another
> module B (in this case the default IO scheduler module)
> - modprobe for B loads the IO scheduler module successfully
> at the end of the module load, it does
> async_synchronize_full() to make sure load_module won't return before
> the module is ready
> *DEADLOCK*, because the async_synchronize_full() thing
> actually waits for not the module B async code (it didn't have any),
> but for the module *A* async code, which is waiting for module B to
> finish.
I think the root problem here, apart from request_module() from block
- which is a bit nasty but making that part completely async would too
be quite nasty albeit in a different way - is that
async_synchronize_full() is way too indescriminate. It's something
only suitable for things like the end of system init.
I'm wondering whether what we need is a rudimentray nesting like the
following.
finished_loading()
{
blah blah;
cookie = async_current_cookie();
do init calls;
async_synchronize_upto(cookie);
blah blah;
}
The nesting here would be an approximation as the dependency recorded
here is chronological. I *suspect* this should be safe unless the
module is doing something weird. Need to think more about it. One
way or the other, I think what we need is some form of scoping for
flushing async ops.
BTW, the current synchronization is broken - cookie isn't transferred
to running->domain in queueing order but __lowest_in_progress()
assumes that. I think I broke that while converting it to workqueue.
Anyways, working on it.
Thanks.
--
tejun
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