Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

[Ming Lei]

On Tue, Nov 02, 2021 at 09:25:44AM -0700, Luis Chamberlain wrote:
> On Tue, Nov 02, 2021 at 04:24:06PM +0100, Petr Mladek wrote:
> > On Wed 2021-10-27 13:57:40, Miroslav Benes wrote:
> > > >From my perspective, it is quite easy to get it wrong due to either a lack 
> > > of generic support, or missing rules/documentation. So if this thread 
> > > leads to "do not share locks between a module removal and a sysfs 
> > > operation" strict rule, it would be at least something. In the same 
> > > manner as Luis proposed to document try_module_get() expectations.
> > 
> > The rule "do not share locks between a module removal and a sysfs
> > operation" is not clear to me.
> 
> That's exactly it. It *is* not. The test_sysfs selftest will hopefully
> help with this. But I'll wait to take a final position on whether or not
> a generic fix should be merged until the Coccinelle patch which looks
> for all uses cases completes.
> 
> So I think that once that Coccinelle hunt is done for the deadlock, we
> should also remind folks of the potential deadlock and some of the rules
> you mentioned below so that if we take a position that we don't support
> this, we at least inform developers why and what to avoid. If Coccinelle
> finds quite a bit of cases, then perhaps evaluating the generic fix
> might be worth evaluating.
> 
> > IMHO, there are the following rules:
> > 
> > 1. rule: kobject_del() or kobject_put() must not be called under a lock that
> > 	 is used by store()/show() callbacks.
> > 
> >    reason: kobject_del() waits until the sysfs interface is destroyed.
> > 	 It has to wait until all store()/show() callbacks are finished.
> 
> Right, this is what actually started this entire conversation.
> 
> Note that as Ming pointed out, the generic kernfs fix I proposed would
> only cover the case when kobject_del() ends up being called on module
> exit, so it would not cover the cases where perhaps kobject_del() might
> be called outside of module exit, and so the cope of the possible
> deadlock then increases in scope.
> 
> Likewise, the Coccinelle hunt I'm trying would only cover the module
> exit case. I'm a bit of afraid of the complexity of a generic hunt
> as expresed in rule 1.

Question is that why one shared lock is required between kobject_del()
and its show()/store(), both zram and livepatch needn't that. Is it
one common usage?

> 
> > 
> > 2. rule: kobject_del()/kobject_put() must not be called from the
> > 	related store() callbacks.
> > 
> >    reason: same as in 1st rule.
> 
> Sensible corollary.
> 
> Given tha the exact kobjet_del() / kobject_put() which must not be
> called from the respective sysfs ops depends on which kobject is
> underneath the device for which the sysfs ops is being created,
> it would make this hunt in Coccinelle a bit tricky. My current iteration
> of a coccinelle hunt cheats and looks at any sysfs looking op and
> ensures a module exit exists.

Actually kernfs/sysfs provides interface for supporting deleting
kobject/attr from the attr's show()/store(), see example of
sdev_store_delete(), and the livepatch example:

https://lore.kernel.org/lkml/20211102145932.3623108-4-ming.lei@xxxxxxxxxx/

> 
> > 3. rule: module_exit() must wait until all release() callbacks are called
> > 	 when kobject are static.
> > 
> >    reason: kobject_put() must be called to clean up internal
> > 	dependencies. The clean up might be done asynchronously
> > 	and need access to the kobject structure.
> 
> This might be an easier rule to implement a respective Coccinelle rule
> for.

If kobject_del() is done in module_exit() or before module_exit(),
kobject should have been freed in module_exit() via kobject_put().

But yes, it can be asynchronously because of CONFIG_DEBUG_KOBJECT_RELEASE,
seems like one real issue.


Thanks,
Ming