workqueue: fix CPU binding of flush_delayed_work[_sync]()

delayed_work encodes the workqueue to use and the last CPU in
delayed_work->work.data while it's on timer.  The target CPU is
implicitly recorded as the CPU the timer is queued on and
delayed_work_timer_fn() queues delayed_work->work to the CPU it is
running on.

Unfortunately, this leaves flush_delayed_work[_sync]() no way to find
out which CPU the delayed_work was queued for when they try to
re-queue after killing the timer.  Currently, it chooses the local CPU
flush is running on.  This can unexpectedly move a delayed_work queued
on a specific CPU to another CPU and lead to subtle errors.

There isn't much point in trying to save several bytes in struct
delayed_work, which is already close to a hundred bytes on 64bit with
all debug options turned off.  This patch adds delayed_work->cpu to
remember the CPU it's queued for.

Note that if the timer is migrated during CPU down, the work item
could be queued to the downed global_cwq after this change.  As a
detached global_cwq behaves like an unbound one, this doesn't change
much for the delayed_work.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 41ae2c0979fe..11723c5b2b20 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1319,7 +1319,7 @@ void delayed_work_timer_fn(unsigned long __data)
         struct cpu_workqueue_struct *cwq = get_work_cwq(&dwork->work);
 
         local_irq_disable();
-        __queue_work(WORK_CPU_UNBOUND, cwq->wq, &dwork->work);
+        __queue_work(dwork->cpu, cwq->wq, &dwork->work);
         local_irq_enable();
 }
 EXPORT_SYMBOL_GPL(delayed_work_timer_fn);
@@ -1356,6 +1356,7 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
 
         set_work_cwq(work, get_cwq(lcpu, wq), 0);
 
+        dwork->cpu = cpu;
         timer->expires = jiffies + delay;
 
         if (unlikely(cpu != WORK_CPU_UNBOUND))
@@ -2997,7 +2998,7 @@ bool flush_delayed_work(struct delayed_work *dwork)
 {
         local_irq_disable();
         if (del_timer_sync(&dwork->timer))
-                __queue_work(WORK_CPU_UNBOUND,
+                __queue_work(dwork->cpu,
                              get_work_cwq(&dwork->work)->wq, &dwork->work);
         local_irq_enable();
         return flush_work(&dwork->work);
@@ -3020,7 +3021,7 @@ bool flush_delayed_work_sync(struct delayed_work *dwork)
 {
         local_irq_disable();
         if (del_timer_sync(&dwork->timer))
-                __queue_work(WORK_CPU_UNBOUND,
+                __queue_work(dwork->cpu,
                              get_work_cwq(&dwork->work)->wq, &dwork->work);
         local_irq_enable();
         return flush_work_sync(&dwork->work);