[PATCH] sysfs: fix deadlock

[Ingo Molnar]

[ Greg, please see the sysfs fix further below. ]

* Nick Piggin <npiggin@xxxxxxx> wrote:

> >  - moved the might_sleep() check outside the in_atomic() check,
> 
> Hmm... but then it has the same failure case again in the is_preempt() 
> code, does it not?
> 
> I guess we should just convert that guy to either use get_user_atomic, 
> (which would mean implementing that for x86), or use 
> copy_from_user_inatomic.

i've done the v3 patch below - that seems to have passed all my testing 
without any new bugs found. I've reinstated your the clear_user() 
might_fault() check, plus i removed it from __[get|put]_user_size, which 
the _inatomic() API variants use. That enabled me to utilize the 
_inatomic() API in probe_kernel_address().

we still have the checks in put_user()/get_user() and in all the 
copy_*_user() APIs, which should be strong enough. [ I havent fully 
checked whether __get_user_size() might be used by some less frequent 
API - if it is then that API should grow a might_fault() check. ]

> > i've attached the config.
> > 
> > at first sight it looks like a genuine bug in fs/sysfs/bin.c?
> 
> Yes, it is a real bug by the looks. bin.c takes bb->mutex under 
> mmap_sem when it is mmapped, and then does its copy_*_user under 
> bb->mutex too.

ok - second patch attached below, Greg, could you please apply? This is 
for v2.6.27 too i think.

> > i.e. your patches are working as expected and the extended 
> > validation mechanism is finding real bugs :-)
> 
> Yeah it's nice. I'm just hoping we don't come across one that is as 
> difficult to fix as prepare_write/commit_write were ;)
> 
> Here is a basic fix for the sysfs lor.

and that did the trick here - the patch with a tidied up changelog is 
attached further below. [ the second patch is standalone and does not 
need the first patch which is relative to tip/master ]

thanks Nick, i think this is a great addition to lockdep! It already 
found two real locking bugs within a day. If you can think of any other 
proactive methods to widen our lock hierarchy knowledge that would be 
great to add. I think what we want is to insert knowledge about other 
unlikely lock acquire events, for locks that have a historic pattern of 
producing regular locking bugs.

	Ingo

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