On 01/15, Srivatsa Vaddagiri wrote:
>
> On Mon, Jan 15, 2007 at 02:54:10AM +0300, Oleg Nesterov wrote:
> > How about the pseudo-code below?
>
> Some quick comments:
>
> - singlethread_cpu needs to be hotplug safe (broken currently)
Why? Could you explain?
> - Any reason why cpu_populated_map is not modified on CPU_DEAD?
Because CPU_DEAD/CPU_UP_CANCELED doesn't wait for cwq->thread to exit.
cpu_populated_map never shrinks, it only grows on CPU_UP_PREPARE.
We can change this, but it needs some more code, and I am not sure
we need it. Note that a "false" bit in cpu_populated_map only means
that flush_work/flush_workqueue/destroy_workqueu will do lock/unlock
of cwq->lock, nothing more.
> - I feel more comfortable if workqueue_cpu_callback were to take
> workqueue_mutex in LOCK_ACQ and release it in LOCK_RELEASE
> notifications.
The whole purpose of this change to avoid this!
Gautham R Shenoy wrote:
>
> Oh! I was refering to the other set of workqueue deadlock woes. The
> ones caused when subsystems (like cpufreq) try to create/destroy
> workqueue from the cpuhotplug callback path.
>
> Creation/destruction of workqueue requires us to take workqueue_mutex,
> which would have already been taken during CPU_LOCK_ACQUIRE.
With this change workqueue_mutex is only taken to protect workqueues
list, why should we hold it for (say) CPU_UP_PREPARE->CPU_ONLINE path?
> This will provide stable access to cpu_populated_map
> to functions like __create_workqueue.
I think this is not needed.
> Finally, I wonder if these changes will be unnecessary if we move to
> process freezer based hotplug locking ...
This change ir not strictly necessary but imho make the code better and
shrinks .text by 379 bytes.
But I believe that freezer will change nothing for workqueue. We still
need take_over_work(), and hacks like migrate_sequence. And no, CPU_DEAD
can't just thaw cwq->thread which was bound to the dead CPU to complete
kthread_stop(), we should thaw all processes.
Oleg.
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