[PATCH 0/1] workqueue: don't migrate pending works from the dead CPU

[Oleg Nesterov <oleg@tv-sign.ru>]

Andrew, please drop the workqueue-rework-threads-hotplug-management.patch,
and please take this one (feel free to just kill me instead).

We have workqueue callbacks (slab for example) which assume that CPU is
stable during the execution since cwq->thread is pinned to a processor.
With or without that patch this is not true with CONFIG_HOTPLUG_CPU, and
the problem is not connected to workqueue.c.

However, my patch makes the things worse, because CPU_UP_PREPARE creates
cwq->thread which is not pinned to CPU until CPU_ONLINE. This is solvable,
but needs a serious complication, so this patch can't be considered as a
cleanup.

The new patch removes take_over_work() and friends (as the old one did),
but retains an unfortunate property of the current implementation: the
workqueue_mutex is held from CPU_LOCK_ACQUIRE to CPU_LOCK_RELEASE, and
CPU_DEAD stops cwq->thread synchronously.

This means that cpu-hotplug callbacks can't create/destroy workqueues
(Gautham needs this?), and work->func() can't share a lock with cpu hotplug.
May be we can change this later, but this needs a separate patch.

Srivatsa, do you have any objections?

The new patch is very similar (in fact, a subset), I don't send incremental
update because this will break changelogs.

Oleg.