Re: [RFC PATCH 8/9] livepatch: allow patch modules to be removed

Josh Poimboeuf
2015-02-17T15:55:57Z

On Mon, Feb 16, 2015 at 05:06:15PM +0100, Miroslav Benes wrote:
> On Fri, 13 Feb 2015, Josh Poimboeuf wrote:
>
> > On Fri, Feb 13, 2015 at 05:17:10PM +0100, Miroslav Benes wrote:
> > > On Fri, 13 Feb 2015, Josh Poimboeuf wrote:
> > > > Hm, even with Jiri Slaby's suggested fix to add the completion to the
> > > > unregister path, I still get a lockdep warning. This looks more insidious,
> > > > related to the locking order of a kernfs lock and the klp lock. I'll need to
> > > > look at this some more...
> > >
> > > Yes, I was afraid of this. Lockdep warning is a separate bug. It is caused
> > > by taking klp_mutex in enabled_store. During rmmod klp_unregister_patch
> > > takes klp_mutex and destroys the sysfs structure. If somebody writes to
> > > enabled just after unregister takes the mutex and before the sysfs
> > > removal, he would cause the deadlock, because enabled_store takes the
> > > "sysfs lock" and then klp_mutex. That is exactly what the lockdep tells us
> > > below.
> > >
> > > We can look for inspiration elsewhere. Grep for s_active through git log
> > > of the mainline offers several commits which dealt exactly with this. Will
> > > browse through that...
> >
> > Thanks Miroslav, please let me know what you find. It wouldn't surprise
> > me if this were a very common problem.
> >
> > One option would be to move the enabled_store() work out to a workqueue
> > or something.
>
> Yes, that is one possibility. It is not the only one.
>
> 1. we could replace mutex_lock in enabled_store with mutex_trylock. If the
> lock was not acquired we would return -EBUSY. Or could we 'return
> restart_syscall' (maybe after some tiny msleep)?

Hm, doesn't that still violate the locking order rules? I thought locks
always had to be taken in the same order -- always sysfs before klp, or
klp before sysfs. Not sure if there would still be any deadlocks
lurking, but lockdep might still complain.

> 2. we could reorganize klp_unregister_patch somehow and move sysfs removal
> out of mutex protection.

Yeah, I was thinking about this too. Pretty sure we'd have to remove
both the sysfs add and the sysfs removal from mutex protection. I like
this option if we can get it to work.

--
Josh