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From:   Mark Hasemeyer <markhas@google.com>
Date:   Thu, 16 Nov 2023 09:29:51 -0700
Message-ID: <CAP0ea-s2QwQhKpu81b+n5Fcq7dscbwTxoFf2tpV926RXw3ca1g@mail.gmail.com>
Subject: Re: [PATCH] cgroup_freezer: cgroup_freezing: Check if not frozen
To:     Tim Van Patten <timvp@chromium.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Tim Van Patten <timvp@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        cgroups@vger.kernel.org
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>
> From: Tim Van Patten <timvp@google.com>
>
> __thaw_task() was recently updated to warn if the task being thawed was
> part of a freezer cgroup that is still currently freezing:
>
>         void __thaw_task(struct task_struct *p)
>         {
>         ...
>                 if (WARN_ON_ONCE(freezing(p)))
>                         goto unlock;
>
> This has exposed a bug in cgroup1 freezing where when CGROUP_FROZEN is
> asserted, the CGROUP_FREEZING bits are not also cleared at the same
> time. Meaning, when a cgroup is marked FROZEN it continues to be marked
> FREEZING as well. This causes the WARNING to trigger, because
> cgroup_freezing() thinks the cgroup is still freezing.
>
> There are two ways to fix this:
>
> 1. Whenever FROZEN is set, clear FREEZING for the cgroup and all
> children cgroups.
> 2. Update cgroup_freezing() to also verify that FROZEN is not set.
>
> This patch implements option (2), since it's smaller and more
> straightforward.
>
> Signed-off-by: Tim Van Patten <timvp@google.com>
> ---
>
>  kernel/cgroup/legacy_freezer.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/cgroup/legacy_freezer.c b/kernel/cgroup/legacy_freezer.c
> index 122dacb3a443..66d1708042a7 100644
> --- a/kernel/cgroup/legacy_freezer.c
> +++ b/kernel/cgroup/legacy_freezer.c
> @@ -66,9 +66,15 @@ static struct freezer *parent_freezer(struct freezer *freezer)
>  bool cgroup_freezing(struct task_struct *task)
>  {
>         bool ret;
> +       unsigned int state;
>
>         rcu_read_lock();
> -       ret = task_freezer(task)->state & CGROUP_FREEZING;
> +       /* Check if the cgroup is still FREEZING, but not FROZEN. The extra
> +        * !FROZEN check is required, because the FREEZING bit is not cleared
> +        * when the state FROZEN is reached.
> +        */
> +       state = task_freezer(task)->state;
> +       ret = (state & CGROUP_FREEZING) && !(state & CGROUP_FROZEN);
>         rcu_read_unlock();
>
>         return ret;
> --
Tested-by: Mark Hasemeyer <markhas@chromium.org>