Q: select_fallback_rq() && cpuset_lock()

[Oleg Nesterov <oleg@redhat.com>]

Hello.

I tried to remove the deadlockable cpuset_lock() many times, but my
attempts were ignored by cpuset maintainers ;)

In particular, see http://marc.info/?l=linux-kernel&m=125261083613103

But now I have another question. Since 5da9a0fb673a0ea0a093862f95f6b89b3390c31e
cpuset_cpus_allowed_locked() is called without callback_mutex held by
try_to_wake_up().

And, without callback_mutex held, isn't it possible to race with, say,
update_cpumask() which changes cpuset->cpus_allowed? Yes, update_tasks_cpumask()
should fixup task->cpus_allowed later. But isn't it possible (at least
in theory) that try_to_wake_up() gets, say, all-zeroes in task->cpus_allowed
after select_fallback_rq()->cpuset_cpus_allowed_locked() if we race with
update_cpumask()->cpumask_copy() ?

Oleg.