Messages in this thread | | | Date | Wed, 9 May 2012 13:09:27 -0700 (PDT) | From | Hugh Dickins <> | Subject | Re: linux-next oops in __lock_acquire for process_one_work |
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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
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