Re: [PATCH v8 00/14] lockdep: Implement crossrelease feature

Byungchul Park
2017-08-16T06:38:57Z

On Wed, Aug 16, 2017 at 01:40:51PM +0800, Boqun Feng wrote:
> > > > Worker A : acquired of wfc.work -> wait for cpu_hotplug_lock to be released
> > > > Task B : acquired of cpu_hotplug_lock -> wait for lock#3 to be released
> > > > Task C : acquired of lock#3 -> wait for completion of barr->done
> > >
> > > >From the stack trace below, this barr->done is for flush_work() in
> > > lru_add_drain_all_cpuslocked(), i.e. for work "per_cpu(lru_add_drain_work)"
> > >
> > > > Worker D : wait for wfc.work to be released -> will complete barr->done
> > >
> > > and this barr->done is for work "wfc.work".
> >
> > I think it can be the same instance. wait_for_completion() in flush_work()
> > e.g. at task C in my example, waits for completion which we expect to be
> > done by a worker e.g. worker D in my example.
> >
> > I think the problem is caused by a write-acquisition of wfc.work in
> > process_one_work(). The acquisition of wfc.work should be reenterable,
> > that is, read-acquisition, shouldn't it?
> >
>
> The only thing is that wfc.work is not a real and please see code in
> flush_work(). And if a task C do a flush_work() for "wfc.work" with
> lock#3 held, it needs to "acquire" wfc.work before it
> wait_for_completion(), which is already a deadlock case:
>
> lock#3 -> wfc.work -> cpu_hotplug_lock -+
> ^ |
> | |
> +-------------------------------------+
>
> , without crossrelease enabled. So the task C didn't flush work wfc.work
> in the previous case, which implies barr->done in Task C and Worker D
> are not the same instance.
>
> Make sense?

Thank you very much for your explanation. I misunderstood how flush_work()
works. Yes, it seems to be led by incorrect class of completion.

Thanks,
Byungchul

>
> Regards,
> Boqun
>
> > I might be wrong... Please fix me if so.
> >
> > Thank you,
> > Byungchul
> >
> > > So those two barr->done could not be the same instance, IIUC. Therefore
> > > the deadlock case is not possible.
> > >
> > > The problem here is all barr->done instances are initialized at
> > > insert_wq_barrier() and they belongs to the same lock class, to fix
> > > this, we need to differ barr->done with different lock classes based on
> > > the corresponding works.
> > >
> > > How about the this(only compilation test):
> > >
> > > ----------------->8
> > > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > > index e86733a8b344..d14067942088 100644
> > > --- a/kernel/workqueue.c
> > > +++ b/kernel/workqueue.c
> > > @@ -2431,6 +2431,27 @@ struct wq_barrier {
> > > struct task_struct *task; /* purely informational */
> > > };
> > >
> > > +#ifdef CONFIG_LOCKDEP_COMPLETE
> > > +# define INIT_WQ_BARRIER_ONSTACK(barr, func, target) \
> > > +do { \
> > > + INIT_WORK_ONSTACK(&(barr)->work, func); \
> > > + __set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&(barr)->work)); \
> > > + lockdep_init_map_crosslock((struct lockdep_map *)&(barr)->done.map, \
> > > + "(complete)" #barr, \
> > > + (target)->lockdep_map.key, 1); \
> > > + __init_completion(&barr->done); \
> > > + barr->task = current; \
> > > +} while (0)
> > > +#else
> > > +# define INIT_WQ_BARRIER_ONSTACK(barr, func, target) \
> > > +do { \
> > > + INIT_WORK_ONSTACK(&(barr)->work, func); \
> > > + __set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&(barr)->work)); \
> > > + init_completion(&barr->done); \
> > > + barr->task = current; \
> > > +} while (0)
> > > +#endif
> > > +
> > > static void wq_barrier_func(struct work_struct *work)
> > > {
> > > struct wq_barrier *barr = container_of(work, struct wq_barrier, work);
> > > @@ -2474,10 +2495,7 @@ static void insert_wq_barrier(struct pool_workqueue *pwq,
> > > * checks and call back into the fixup functions where we
> > > * might deadlock.
> > > */
> > > - INIT_WORK_ONSTACK(&barr->work, wq_barrier_func);
> > > - __set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));
> > > - init_completion(&barr->done);
> > > - barr->task = current;
> > > + INIT_WQ_BARRIER_ONSTACK(barr, wq_barrier_func, target);
> > >
> > > /*
> > > * If @target is currently being executed, schedule the