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Date:   Tue, 18 Apr 2023 11:18:26 -0300
From:   Wander Lairson Costa <wander@redhat.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Kees Cook <keescook@chromium.org>,
        Christian Brauner <brauner@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Shakeel Butt <shakeelb@google.com>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-perf-users@vger.kernel.org>, Hu Chunyu <chuhu@redhat.com>,
        Paul McKenney <paulmck@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe()
 function
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References: <20230414125532.14958-1-wander@redhat.com>
 <20230414125532.14958-3-wander@redhat.com>
 <e197ad4d-a60b-f773-dd74-ba91ad66a617@redhat.com>
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On Mon, Apr 17, 2023 at 02:51:45PM -0400, Waiman Long wrote:
> 
> On 4/14/23 08:55, Wander Lairson Costa wrote:
> > Due to the possibility of indirectly acquiring sleeping locks, it is
> > unsafe to call put_task_struct() in atomic contexts when the kernel is
> > compiled with PREEMPT_RT.
> > 
> > To mitigate this issue, this commit introduces
> > put_task_struct_atomic_safe(), which schedules __put_task_struct()
> > through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
> > be a more natural approach, we cannot allocate dynamic memory from
> > atomic context in PREEMPT_RT, making the code more complex.
> > 
> > This implementation ensures safe execution in atomic contexts and
> > avoids any potential issues that may arise from using the non-atomic
> > version.
> > 
> > Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> > Reported-by: Hu Chunyu <chuhu@redhat.com>
> > Cc: Paul McKenney <paulmck@kernel.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > ---
> >   include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
> >   kernel/fork.c              |  8 ++++++++
> >   2 files changed, 39 insertions(+)
> > 
> > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > index b597b97b1f8f..5c13b83d7008 100644
> > --- a/include/linux/sched/task.h
> > +++ b/include/linux/sched/task.h
> > @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
> >   void put_task_struct_rcu_user(struct task_struct *task);
> > +extern void __delayed_put_task_struct(struct rcu_head *rhp);
> > +
> > +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> > +{
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +		/*
> > +		 * Decrement the refcount explicitly to avoid unnecessarily
> > +		 * calling call_rcu.
> > +		 */
> > +		if (refcount_dec_and_test(&task->usage))
> > +			/*
> > +			 * under PREEMPT_RT, we can't call put_task_struct
> > +			 * in atomic context because it will indirectly
> > +			 * acquire sleeping locks.
> > +			 * call_rcu() will schedule delayed_put_task_struct_rcu()
> delayed_put_task_struct_rcu()?
> > +			 * to be called in process context.
> > +			 *
> > +			 * __put_task_struct() is called called when
> "called called"?
> > +			 * refcount_dec_and_test(&t->usage) succeeds.
> > +			 *
> > +			 * This means that it can't "conflict" with
> > +			 * put_task_struct_rcu_user() which abuses ->rcu the same
> > +			 * way; rcu_users has a reference so task->usage can't be
> > +			 * zero after rcu_users 1 -> 0 transition.
> 
> Note that put_task_struct_rcu_user() isn't the only user of task->rcu.
> delayed_free_task() in kernel/fork.c also uses it, though it is only called
> in the error case. Still you may need to take a look to make sure that there
> is no conflict.
> 

delayed_free_task() is called when a process fails to start. Therefore, AFAICT,
there is no way it can conflict with put_task_struct().

> Cheers,
> Longman
>