Messages in this thread | | | Date | Tue, 18 Apr 2023 11:18:26 -0300 | From | Wander Lairson Costa <> | Subject | Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function |
| |
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 >
| |