Re: linux-next oops in __lock_acquire for process_one_work

[Hugh Dickins <hughd@google.com>]

On Wed, 9 May 2012, Ingo Molnar wrote:
> * Hugh Dickins <hughd@google.com> wrote:
> 
> > I'll set it going when I get home later - thanks.

Going fine so far, but a more convincing final report tomorrow.

> 
> Do we still need an explanation about why it's needed and why it 
> makes a difference?

I don't see the difficulty in understanding it.  Peter didn't comment
whether my further explanations convinced him or not.  Or perhaps you're
asking for some commit description text - I may not be the right person to
write it, since I didn't make myself understood very well, but here's a go.

lockdep: fix oops in processing workqueue

Under memory load, on x86_64, with lockdep enabled, the workqueue's
process_one_work() has been seen to oops in __lock_acquire(), barfing
on a 0xffffffff00000000 pointer in the lockdep_map's class_cache[].

Because it's permissible to free a work_struct from its callout function,
the map used is an onstack copy of the map given in the work_struct: and
that copy is made without any locking.

Surprisingly, gcc (4.5.1 in Hugh's case) uses "rep movsl" rather than
"rep movsq" for that structure copy: which might race with a workqueue
user's wait_on_work() doing lock_map_acquire() on the source of the
copy, putting a pointer into the class_cache[], but only in time for
the top half of that pointer to be copied to the destination map.

Boom when process_one_work() subsequently does lock_map_acquire()
on its onstack copy of the lockdep_map.

Fix this, and a similar instance in call_timer_fn(), with a
lockdep_copy_map() function which additionally NULLs the class_cache[].

Note: this oops was actually seen on 3.4-next, where flush_work() newly
does the racing lock_map_acquire(); but Tejun points out that 3.4 and
earlier are already vulnerable to the same through wait_on_work().

Hugh