Message-ID: <1351376558.21585.1.camel@deadeye.wl.decadent.org.uk>
Subject: Re: [PATCH v2 1/1] freezer: change ptrace_stop/do_signal_stop to
 use freezable_schedule()
From: Ben Hutchings <ben@decadent.org.uk>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>, rjw@sisk.pl, linux-kernel@vger.kernel.org,
        lizefan@huawei.com, containers@lists.linux-foundation.org,
        cgroups@vger.kernel.org, stable@vger.kernel.org
Date: Sat, 27 Oct 2012 23:22:38 +0100
In-Reply-To: <20121026174606.GB21639@redhat.com>
References: <20121022174404.GA21553@redhat.com>
	 <20121022211317.GD5951@atj.dyndns.org> <20121023153919.GA16201@redhat.com>
	 <20121024185710.GA12182@atj.dyndns.org> <20121025163941.GA3801@redhat.com>
	 <20121025163959.GB3801@redhat.com> <20121025171812.GE11442@htj.dyndns.org>
	 <20121025173433.GA7650@redhat.com> <20121025173632.GI11442@htj.dyndns.org>
	 <20121026174545.GA21639@redhat.com> <20121026174606.GB21639@redhat.com>
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On Fri, 2012-10-26 at 19:46 +0200, Oleg Nesterov wrote:
> try_to_freeze_tasks() and cgroup_freezer rely on scheduler locks
> to ensure that a task doing STOPPED/TRACED -> RUNNING transition
> can't escape freezing. This mostly works, but ptrace_stop() does
> not necessarily call schedule(), it can change task->state back to
> RUNNING and check freezing() without any lock/barrier in between.
>=20
> We could add the necessary barrier, but this patch changes
> ptrace_stop() and do_signal_stop() to use freezable_schedule().
> This fixes the race, freezer_count() and freezer_should_skip()
> carefully avoid the race.
>=20
> And this simplifies the code, try_to_freeze_tasks/update_if_frozen
> no longer need to use task_is_stopped_or_traced() checks with the
> non trivial assumptions. We can rely on the mechanism which was
> specially designed to mark the sleeping task as "frozen enough".
>=20
> v2: As Tejun pointed out, we can also change get_signal_to_deliver()
> and move try_to_freeze() up before 'relock' label.
>=20
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
[...]

This is not the correct way to submit a change to stable.  Please see
Documentation/stable_kernel_rules.txt

Ben.

--=20
Ben Hutchings
Never attribute to conspiracy what can adequately be explained by stupidity=
.

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