Messages in this thread | | | Date | Mon, 17 Apr 2023 14:51:45 -0400 | Subject | Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function | From | Waiman Long <> |
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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.
Cheers, Longman
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