Under PREEMPT_RT, __put_task_struct() indirectly acquires sleeping
locks. Therefore, it can't be called from an non-preemptible context.
One practical example is splat inside inactive_task_timer(), which is
called in a interrupt context:
CPU: 1 PID: 2848 Comm: life Kdump: loaded Tainted: G W ---------
Hardware name: HP ProLiant DL388p Gen8, BIOS P70 07/15/2012
Call Trace:
dump_stack_lvl+0x57/0x7d
mark_lock_irq.cold+0x33/0xba
? stack_trace_save+0x4b/0x70
? save_trace+0x55/0x150
mark_lock+0x1e7/0x400
mark_usage+0x11d/0x140
__lock_acquire+0x30d/0x930
lock_acquire.part.0+0x9c/0x210
? refill_obj_stock+0x3d/0x3a0
? rcu_read_lock_sched_held+0x3f/0x70
? trace_lock_acquire+0x38/0x140
? lock_acquire+0x30/0x80
? refill_obj_stock+0x3d/0x3a0
rt_spin_lock+0x27/0xe0
? refill_obj_stock+0x3d/0x3a0
refill_obj_stock+0x3d/0x3a0
? inactive_task_timer+0x1ad/0x340
kmem_cache_free+0x357/0x560
inactive_task_timer+0x1ad/0x340
? switched_from_dl+0x2d0/0x2d0
__run_hrtimer+0x8a/0x1a0
__hrtimer_run_queues+0x91/0x130
hrtimer_interrupt+0x10f/0x220
__sysvec_apic_timer_interrupt+0x7b/0xd0
sysvec_apic_timer_interrupt+0x4f/0xd0
? asm_sysvec_apic_timer_interrupt+0xa/0x20
asm_sysvec_apic_timer_interrupt+0x12/0x20
RIP: 0033:0x7fff196bf6f5
Instead of calling __put_task_struct() directly, we defer it using
call_rcu(). A more natural approach would use a workqueue, but since
in PREEMPT_RT, we can't allocate dynamic memory from atomic context,
the code would become more complex because we would need to put the
work_struct instance in the task_struct and initialize it when we
allocate a new task_struct.
Signed-off-by: Wander Lairson Costa <[email protected]>
Reported-by: Hu Chunyu <[email protected]>
Suggested-by: Oleg Nesterov <[email protected]>
Suggested-by: Valentin Schneider <[email protected]>
Cc: Paul McKenney <[email protected]>
---
kernel/fork.c | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index 9f7fe3541897..b2d0d62c9b9d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -840,7 +840,7 @@ static inline void put_signal_struct(struct signal_struct *sig)
free_signal_struct(sig);
}
-void __put_task_struct(struct task_struct *tsk)
+static void ___put_task_struct(struct task_struct *tsk)
{
WARN_ON(!tsk->exit_state);
WARN_ON(refcount_read(&tsk->usage));
@@ -857,6 +857,26 @@ void __put_task_struct(struct task_struct *tsk)
sched_core_free(tsk);
free_task(tsk);
}
+
+static void __delayed_put_task_struct(struct rcu_head *rhp)
+{
+ struct task_struct *task = container_of(rhp, struct task_struct, rcu);
+
+ ___put_task_struct(task);
+}
+
+void __put_task_struct(struct task_struct *tsk)
+{
+ if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
+ /*
+ * under PREEMPT_RT, we can't call put_task_struct
+ * in atomic context because it will indirectly
+ * acquire sleeping locks.
+ */
+ call_rcu(&tsk->rcu, __delayed_put_task_struct);
+ else
+ ___put_task_struct(tsk);
+}
EXPORT_SYMBOL_GPL(__put_task_struct);
void __init __weak arch_task_cache_init(void) { }
--
2.39.1
On 02/01, Wander Lairson Costa wrote:
>
> Instead of calling __put_task_struct() directly, we defer it using
> call_rcu(). A more natural approach would use a workqueue, but since
> in PREEMPT_RT, we can't allocate dynamic memory from atomic context,
> the code would become more complex because we would need to put the
> work_struct instance in the task_struct and initialize it when we
> allocate a new task_struct.
I don't think I can ack the changes in PREEMPT_RT but this version LGTM.
just a couple of purely cosmetic nits, feel free to ignore...
> +static void __delayed_put_task_struct(struct rcu_head *rhp)
> +{
> + struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> +
> + ___put_task_struct(task);
> +}
We already have delayed_put_task_struct() which differs very much.
Perhaps something like ___put_task_struct() will look less confusing.
> +void __put_task_struct(struct task_struct *tsk)
> +{
> + if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
> + /*
> + * under PREEMPT_RT, we can't call put_task_struct
> + * in atomic context because it will indirectly
> + * acquire sleeping locks.
> + */
> + call_rcu(&tsk->rcu, __delayed_put_task_struct);
Perhaps this deserves additional note to explain why is it safe to use tsk->rcu
union. May be this is obvious, but I was confused when I looked at the previous
version ;)
but again, feel free to ignore.
Oleg.
On Thu, Feb 2, 2023 at 3:37 PM, Oleg Nesterov <[email protected]> wrote:
>
> On 02/01, Wander Lairson Costa wrote:
> >
> > Instead of calling __put_task_struct() directly, we defer it using
> > call_rcu(). A more natural approach would use a workqueue, but since
> > in PREEMPT_RT, we can't allocate dynamic memory from atomic context,
> > the code would become more complex because we would need to put the
> > work_struct instance in the task_struct and initialize it when we
> > allocate a new task_struct.
>
> I don't think I can ack the changes in PREEMPT_RT but this version LGTM.
>
>
>
>
> just a couple of purely cosmetic nits, feel free to ignore...
>
> > +static void __delayed_put_task_struct(struct rcu_head *rhp)
> > +{
> > + struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > +
> > + ___put_task_struct(task);
> > +}
>
> We already have delayed_put_task_struct() which differs very much.
> Perhaps something like ___put_task_struct() will look less confusing.
>
___put_task_struct()? I already added a function with this name below.
> > +void __put_task_struct(struct task_struct *tsk)
> > +{
> > + if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
> > + /*
> > + * under PREEMPT_RT, we can't call put_task_struct
> > + * in atomic context because it will indirectly
> > + * acquire sleeping locks.
> > + */
> > + call_rcu(&tsk->rcu, __delayed_put_task_struct);
>
> Perhaps this deserves additional note to explain why is it safe to use tsk->rcu
> union. May be this is obvious, but I was confused when I looked at the previous
> version ;)
>
Makes sense, I will add it in the next version.
> but again, feel free to ignore.
>
> Oleg.
>
On 02/02, Wander Lairson Costa wrote:
>
> > We already have delayed_put_task_struct() which differs very much.
> > Perhaps something like ___put_task_struct() will look less confusing.
> >
>
> ___put_task_struct()? I already added a function with this name below.
Ah, I meant ___put_task_struct_rcu() or something like this. Bug again
this is just cosmetic nit, please ignore
> > > +void __put_task_struct(struct task_struct *tsk)
> > > +{
> > > + if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
> > > + /*
> > > + * under PREEMPT_RT, we can't call put_task_struct
> > > + * in atomic context because it will indirectly
> > > + * acquire sleeping locks.
> > > + */
> > > + call_rcu(&tsk->rcu, __delayed_put_task_struct);
> >
> > Perhaps this deserves additional note to explain why is it safe to use tsk->rcu
> > union. May be this is obvious, but I was confused when I looked at the previous
> > version ;)
> >
>
> Makes sense, I will add it in the next version.
Thanks ;)
Oleg.