Re: [PATCH] padata: fix lockdep warning in padata serialization

[Steffen Klassert]

On Wed, Sep 21, 2022 at 02:51:38PM -0400, Daniel Jordan wrote:
> On Wed, Sep 21, 2022 at 09:36:16AM +0200, Steffen Klassert wrote:
> > On Tue, Sep 20, 2022 at 10:10:57AM -0400, Daniel Jordan wrote:
> > > Yeah, padata_do_serial can be called with BHs off, like in the tipc
> > > stack, but there are also cases where BHs can be on, like lockdep said
> > > here:
> > 
> > padata_do_serial was designed to run with BHs off, it is a bug if it
> > runs with BHs on. But I don't see a case where this can happen. The
> > only user of padata_do_serial is pcrypt in its serialization callbacks
> > (pcrypt_aead_enc, pcrypt_aead_dec) and the async crypto callback
> > pcrypt_aead_done. pcrypt_aead_enc and pcrypt_aead_dec are issued via
> > the padata_serial_worker with the padata->serial call. BHs are
> > off here. The crypto callback also runs with BHs off.
> > 
> > What do I miss here?
> 
> Ugh.. this newer, buggy part of padata_do_parallel:
> 
>   /* Maximum works limit exceeded, run in the current task. */
>   padata->parallel(padata);

Oh well...

> This skips the usual path in padata_parallel_worker, which disables BHs.
> They should be left off in the above case too.
> 
> What about this?
> 
> ---8<---
> 
> Subject: [PATCH] padata: always leave BHs disabled when running ->parallel()
> 
> A deadlock can happen when an overloaded system runs ->parallel() in the
> context of the current task:
> 
>     padata_do_parallel
>       ->parallel()
>         pcrypt_aead_enc/dec
>           padata_do_serial
>             spin_lock(&reorder->lock) // BHs still enabled
>               <interrupt>
>                 ...
>                   __do_softirq
>                     ...
>                       padata_do_serial
>                         spin_lock(&reorder->lock)
> 
> It's a bug for BHs to be on in _do_serial as Steffen points out, so
> ensure they're off in the "current task" case like they are in
> padata_parallel_worker to avoid this situation.
> 
> Reported-by: syzbot+bc05445bc14148d51915@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 4611ce224688 ("padata: allocate work structures for parallel jobs from a pool")
> Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>

Yes, that makes sense.

Acked-by: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>

But we also should look at the call to padata_find_next where BHs are
on. padata_find_next takes the same lock as padata_do_serial, so this
might be a candidate for a deadlock too.