Re: 2.6.24-rc2-mm1: kcryptd vs lockdep

["Torsten Kaiser" <just.for.lkml@googlemail.com>]

On Nov 19, 2007 10:00 PM, Milan Broz <mbroz@redhat.com> wrote:
> Torsten Kaiser wrote:
> > On Nov 19, 2007 8:56 AM, Ingo Molnar <mingo@elte.hu> wrote:
> >> * Torsten Kaiser <just.for.lkml@googlemail.com> wrote:
> ...
> > Above this acquire/release sequence is the following comment:
> > #ifdef CONFIG_LOCKDEP
> >                 /*
> >                  * It is permissible to free the struct work_struct
> >                  * from inside the function that is called from it,
> >                  * this we need to take into account for lockdep too.
> >                  * To avoid bogus "held lock freed" warnings as well
> >                  * as problems when looking into work->lockdep_map,
> >                  * make a copy and use that here.
> >                  */
> >                 struct lockdep_map lockdep_map = work->lockdep_map;
> > #endif
> >
> > Did something trigger this anyway?
> >
> > Anything I could try, apart from more boots with slub_debug=F?
>
> Please could you try which patch from the dm-crypt series cause this ?
> (agk-dm-dm-crypt* names.)
>
> I suspect agk-dm-dm-crypt-move-bio-submission-to-thread.patch because
> there is one work struct used subsequently in two threads...
> (io thread already started while crypt thread is processing lockdep_map
> after calling f(work)...)
>
> (btw these patches prepare dm-crypt for next patchset introducing
> async cryptoapi, so there should be no functional changes yet.)

I looked at all of these agk-*-patches, as the error is not
bisectable, because it triggers unreliable.
The one that looks suspicious is agk-dm-dm-crypt-tidy-io-ref-counting.patch

This one does a functional change, as there now is an additional ref
on io->pending. Instead of only increasing io->pending if there really
are more then one clone-bio, it will now take an additional ref in
crypt_write_io_process().

I certainly agree with the cleanup, but this introduces the following change:

Before the cleanup *all* calls to crypt_dec_pending() was via crypt_endio().
Now there is an additional call to crypt_dec_pending() to balance the
additional ref placed into crypt_write_io_process(). And that one is
not called from whatever context/thread cleans up after
make_generic_request, but directly in the context/thread of the caller
of crypt_write_io_process(), and that is kcryptd.

So now it is possible (if all requests finish before
crypt_write_io_process() returns) that kcryptd itself will release the
bio, but the workqueue infrastructure still seems to have a lock on
that.

But as the comment in run_workqueue says, this should be legal, and I
can't figure out what would make the the lockdep copy mechanism fail.
Especially if the trigger was really a WRITE request, as with
agk-dm-dm-crypt-move-bio-submission-to-thread.patch reverted this
should never use the kcrypt_io-workqueue and so there should be not
even the problem with using INIT_WORK twice on the same work_struct.

... or I just don't see the bug.

Torsten
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