Messages in this thread | | | Date | Mon, 24 Apr 2023 11:09:19 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function |
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On Fri, Apr 14, 2023 at 09:55:28AM -0300, 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() > + * to be called in process context. > + * > + * __put_task_struct() is called called when > + * 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. > + */ > + call_rcu(&task->rcu, __delayed_put_task_struct);
This will invoke __delayed_put_task_struct() with softirqs disabled. Or do softirq-disabled contexts count as non-atomic in PREEMPT_RT?
Thanx, Paul
> + } else { > + put_task_struct(task); > + } > +} > + > /* Free all architecture-specific resources held by a thread. */ > void release_thread(struct task_struct *dead_task); > > diff --git a/kernel/fork.c b/kernel/fork.c > index 0c92f224c68c..9884794fe4b8 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -854,6 +854,14 @@ void __put_task_struct(struct task_struct *tsk) > } > EXPORT_SYMBOL_GPL(__put_task_struct); > > +void __delayed_put_task_struct(struct rcu_head *rhp) > +{ > + struct task_struct *task = container_of(rhp, struct task_struct, rcu); > + > + __put_task_struct(task); > +} > +EXPORT_SYMBOL_GPL(__delayed_put_task_struct); > + > void __init __weak arch_task_cache_init(void) { } > > /* > -- > 2.39.2 >
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