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SubjectRe: [PATCH] cgroup/cpuset: fix circular locking dependency
On Mon, Jan 08, 2018 at 04:28:23AM -0800, Tejun Heo wrote:
> Hello, Paul.
> Sorry about the delay. Travel followed by cold. :(
> On Tue, Jan 02, 2018 at 10:01:19AM -0800, Paul E. McKenney wrote:
> > Actually, after taking a quick look, could you please supply me with
> > a way of mark a statically allocated workqueue as WQ_MEM_RECLAIM after
> > the fact? Otherwise, I end up having to check for the workqueue having
> Hmmm... there is no statically allocated workqueue tho. If you're
> referring to the system-wide workqueues (system*_wq), they're just
> created dynamically early during boot.

Good point, I was confused. But yes, they are conveniently allocated
just before the call to rcu_init(), which does work out well. ;-)

> > been allocated pretty much each time I use it, which is going to be an
> > open invitation for bugs. Plus it looks like there are ways that RCU's
> > workqueue wakeups can be executed during very early boot, which can be
> > handled, but again in a rather messy fashion.
> >
> > In contrast, given a way of mark a statically allocated workqueue
> > as WQ_MEM_RECLAIM after the fact, I simply continue initializing the
> > workqueue at early boot, and then add the WQ_MEM_RECLAIM marking some
> > arbitrarily chosen time after the scheduler has been initialized.
> >
> > The required change to workqueues looks easy, just move the body of
> > the "if (flags & WQ_MEM_RECLAIM) {" statement in __alloc_workqueue_key()
> > to a separate function, right?
> Ah, okay, yes, currently, workqueue init is kinda silly in that while
> it allows init of non-mem-reclaiming workqueues way before workqueue
> is actually online, it doesn't allow the same for mem-reclaiming ones.
> As you pointed out, it's just an oversight on my part as the init path
> split was done initially to accomodate early init of system
> workqueues.
> I'll update the code so that rescuers can be added later too; however,
> please note that while the work items may be queued, they won't be
> executed until workqueue_init() is run (the same as now) as there
> can't be worker threads anyway before that point.

Thank you! I added the following patch to allow RCU access to the
init_rescuer() function. Does that work for you, or did you have some
other arrangement in mind?

Thanx, Paul


commit 66683a07503d71e5d5cceac72caf772e6e59c787
Author: Paul E. McKenney <>
Date: Mon Jan 8 14:27:46 2018 -0800

workqueue: Allow init_rescuer() to be invoked from other files

This commit exports init_rescuer() so that RCU can invoke it.

Signed-off-by: Paul E. McKenney <>
Cc: Tejun Heo <>
Cc: Lai Jiangshan <>

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 4a54ef96aff5..31ce9343b4a9 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -433,6 +433,8 @@ __alloc_workqueue_key(const char *fmt, unsigned int flags, int max_active,
#define create_singlethread_workqueue(name) \
alloc_ordered_workqueue("%s", __WQ_LEGACY | WQ_MEM_RECLAIM, name)

+int init_rescuer(struct workqueue_struct *wq);
extern void destroy_workqueue(struct workqueue_struct *wq);

struct workqueue_attrs *alloc_workqueue_attrs(gfp_t gfp_mask);
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c86cc1ed678b..7440c61c6213 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3943,7 +3943,7 @@ static int wq_clamp_max_active(int max_active, unsigned int flags,
* Workqueues which may be used during memory reclaim should have a rescuer
* to guarantee forward progress.
-static int init_rescuer(struct workqueue_struct *wq)
+int init_rescuer(struct workqueue_struct *wq)
struct worker *rescuer;
int ret;
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