Re: [Patch] workqueue: move lockdep annotations up to destroy_workqueue()

From: Cong Wang
Date: Thu Apr 01 2010 - 00:24:59 EST

Tejun Heo wrote:

On 04/01/2010 01:09 PM, Cong Wang wrote:
This seems to be from the original thread of frame#3. It's grabbing
wq lock here but the problem is that the lock will be released
immediately, so bond_dev->name (the wq) can't be held by the time it
reaches frame#3. How is this dependency chain completed? Is it
somehow transitive through rtnl_mutex?
wq lock is held *after* cpu_add_remove_lock, lockdep also said this,
the process is trying to hold wq lock while having cpu_add_remove_lock.

Yeah yeah, I'm just failing to see how the other direction is
completed. ie. where does the kernel try to grab cpu_add_remove_lock
*after* grabbing wq lock?

Isn't there a circular dependency here? bonding_exit() calls
destroy_workqueue() under rtnl_mutex but destroy_workqueue() should
flush works which could be trying to grab rtnl_lock. Or am I
completely misunderstanding locking here?
Sure, that is why I sent another patch for bonding. :)

Ah... great. :-)

After this patch, another lockdep warning appears, it is exactly what
you expect.

Hmmm... can you please try to see whether this circular locking
warning involving wq->lockdep_map is reproducible w/ the bonding
locking fixed? I still can't see where wq -> cpu_add_remove_lock
dependency is created.

I thought this is obvious.

Here it is:

void destroy_workqueue(struct workqueue_struct *wq)
const struct cpumask *cpu_map = wq_cpu_map(wq);
int cpu;

cpu_maps_update_begin(); <----------------- Hold cpu_add_remove_lock here

for_each_cpu(cpu, cpu_map)
cleanup_workqueue_thread(per_cpu_ptr(wq->cpu_wq, cpu)); <------ See below
cpu_maps_update_done(); <----------------- Release cpu_add_remove_lock here

static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq)
* Our caller is either destroy_workqueue() or CPU_POST_DEAD,
* cpu_add_remove_lock protects cwq->thread.
if (cwq->thread == NULL)

lock_map_acquire(&cwq->wq->lockdep_map); <-------------- Lockdep complains here.

Am I missing something??

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