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

Byungchul Park
2017-08-16T04:39:09Z

On Wed, Aug 16, 2017 at 12:05:31PM +0800, Boqun Feng wrote:
> On Wed, Aug 16, 2017 at 09:16:37AM +0900, Byungchul Park wrote:
> > On Tue, Aug 15, 2017 at 10:20:20AM +0200, Ingo Molnar wrote:
> > >
> > > So with the latest fixes there's a new lockdep warning on one of my testboxes:
> > >
> > > [ 11.322487] EXT4-fs (sda2): mounted filesystem with ordered data mode. Opts: (null)
> > >
> > > [ 11.495661] ======================================================
> > > [ 11.502093] WARNING: possible circular locking dependency detected
> > > [ 11.508507] 4.13.0-rc5-00497-g73135c58-dirty #1 Not tainted
> > > [ 11.514313] ------------------------------------------------------
> > > [ 11.520725] umount/533 is trying to acquire lock:
> > > [ 11.525657] ((complete)&barr->done){+.+.}, at: [<ffffffff810fdbb3>] flush_work+0x213/0x2f0
> > > [ 11.534411]
> > > but task is already holding lock:
> > > [ 11.540661] (lock#3){+.+.}, at: [<ffffffff8122678d>] lru_add_drain_all_cpuslocked+0x3d/0x190
> > > [ 11.549613]
> > > which lock already depends on the new lock.
> > >
> > > The full splat is below. The kernel config is nothing fancy - distro derived,
> > > pretty close to defconfig, with lockdep enabled.
> >
> > I see...
> >
> > 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?

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