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

Boqun Feng
2017-08-16T04:05:16Z

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".

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