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Subject[PATCH 3.16 244/357] workqueue: apply __WQ_ORDERED to create_singlethread_workqueue()
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3.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tejun Heo <tj@kernel.org>

commit e09c2c295468476a239d13324ce9042ec4de05eb upstream.

create_singlethread_workqueue() is a compat interface for single
threaded workqueue which maps to ordered workqueue w/ rescuer in the
current implementation. create_singlethread_workqueue() currently
implemented by invoking alloc_workqueue() w/ appropriate parameters.

8719dceae2f9 ("workqueue: reject adjusting max_active or applying
attrs to ordered workqueues") introduced __WQ_ORDERED to protect
ordered workqueues against dynamic attribute changes which can break
ordering guarantees but forgot to apply it to
create_singlethread_workqueue(). This in itself is okay as nobody
currently uses dynamic attribute change on workqueues created with
create_singlethread_workqueue().

However, 4c16bd327c ("workqueue: implement NUMA affinity for unbound
workqueues") broke singlethreaded guarantee for ordered workqueues
through allocating a separate pool_workqueue on each NUMA node by
default. A later change 8a2b75384444 ("workqueue: fix ordered
workqueues in NUMA setups") fixed it by allocating only one global
pool_workqueue if __WQ_ORDERED is set.

Combined, the __WQ_ORDERED omission in create_singlethread_workqueue()
became critical breaking its single threadedness and ordering
guarantee.

Let's make create_singlethread_workqueue() wrap
alloc_ordered_workqueue() instead so that it inherits __WQ_ORDERED and
can implicitly track future ordered_workqueue changes.

v2: I missed that __WQ_ORDERED now protects against pwq splitting
across NUMA nodes and incorrectly described the patch as a
nice-to-have fix to protect against future dynamic attribute
usages. Oleg pointed out that this is actually a critical
breakage due to 8a2b75384444 ("workqueue: fix ordered workqueues
in NUMA setups").

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Mike Anderson <mike.anderson@us.ibm.com>
Cc: Oleg Nesterov <onestero@redhat.com>
Cc: Gustavo Luiz Duarte <gduarte@redhat.com>
Cc: Tomas Henzl <thenzl@redhat.com>
Fixes: 4c16bd327c ("workqueue: implement NUMA affinity for unbound workqueues")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
include/linux/workqueue.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -419,7 +419,7 @@ __alloc_workqueue_key(const char *fmt, u
alloc_workqueue("%s", WQ_FREEZABLE | WQ_UNBOUND | WQ_MEM_RECLAIM, \
1, (name))
#define create_singlethread_workqueue(name) \
- alloc_workqueue("%s", WQ_UNBOUND | WQ_MEM_RECLAIM, 1, (name))
+ alloc_ordered_workqueue("%s", WQ_MEM_RECLAIM, name)

extern void destroy_workqueue(struct workqueue_struct *wq);




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