On Sun, Jan 07, 2007 at 03:56:03PM +0300, Oleg Nesterov wrote:
> Srivatsa, I'm completely new to cpu-hotplug, so please correct me if I'm
> wrong (in fact I _hope_ I am wrong) but as I see it, the hotplug/workqueue
> interaction is broken by design, it can't be fixed by changing just locking.
>
> Once again. CPU dies, CPU_DEAD calls kthread_stop() and sleeps until
> cwq->thread exits. To do so, this thread must at least complete the
> currently running work->func().
If run_workqueue() takes a lock_cpu_hotplug() successfully, then we shouldnt
even reach till this point, as it will block writers (cpu_down/up) until it
completes.
run_workqueue()
---------------
try_again:
rc = lock_cpu_hotplug_interruptible();
if (rc && kthread_should_stop())
return;
if (rc != 0)
goto try_again;
/* cpu_down/up shouldnt happen now untill we call unlock_cpu_hotplug */
while (!list_empty(..))
work->func();
unlock_cpu_hotplug();
If work->func() calls something (say flush_workqueue()) which requires a
lock_cpu_hotplug() again, there are two ways to support it:
Method 1: Add a field, hotplug_lock_held, in task_struct
If current->hotplug_lock_held > 1, then lock_cpu_hotplug()
merely increments it and returns success. Its counterpart,
unlock_cpu_hotplug() will decrement the count.
Easiest to implement. However additional field is required in
each task_struct, which may not be attractive for some.
Method 2 : Bias readers over writers:
This method will support recursive calls to lock_cpu_hotplug()
by the same thread, w/o requiring a field in task_struct. To
accomplish this, readers are biased over writers i.e
reader1_lock(); <- success
writer1_lock(); <- blocks on reader1
reader2_lock(); <- success
A fair lock would have blocked reader2_lock() until
writer1_lock()/writer1_unlock() is complete, but since we are required to
support recursion w/o maintaining a task_struct field, we let reader2_lock()
succeed, even though it could be from a different thread.
> Andrew, Ingo, this also means that freezer can't solve this particular
> problem either (if i am right).
freezer wont give stable access to cpu_online_map either, as could typically be
required in functions like flush_workqueue.
--
Regards,
vatsa
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