linux-kernel - Q: select_fallback_rq() && cpuset

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Message-ID: <20100309180615.GA11681@redhat.com>
Date:	Tue, 9 Mar 2010 19:06:15 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>, Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Q: select_fallback_rq() && cpuset_lock()

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.

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