lkml.org 
[lkml]   [2021]   [Jul]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability
On Wed, Jul 21, 2021 at 12:51:17PM +0100, Valentin Schneider wrote:
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
> kernel/rcu/tree_plugin.h | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index ad0156b86937..6c3c4100da83 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -70,8 +70,7 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
> !(lockdep_is_held(&rcu_state.barrier_mutex) ||
> (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
> rcu_lockdep_is_held_nocb(rdp) ||
> - (rdp == this_cpu_ptr(&rcu_data) &&
> - !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
> + (rdp == this_cpu_ptr(&rcu_data) && is_pcpu_safe()) ||

I fear that won't work. We really need any caller of rcu_rdp_is_offloaded()
on the local rdp to have preemption disabled and not just migration disabled,
because we must protect against concurrent offloaded state changes.

The offloaded state is changed by a workqueue that executes on the target rdp.

Here is a practical example where it matters:

CPU 0
-----
// =======> task rcuc running
rcu_core {
rcu_nocb_lock_irqsave(rdp, flags) {
if (!rcu_segcblist_is_offloaded(rdp->cblist)) {
// is not offloaded right now, so it's going
// to just disable IRQs. Oh no wait:
// preemption
// ========> workqueue running
rcu_nocb_rdp_offload();
// ========> task rcuc resume
local_irq_disable();
}
}
....
rcu_nocb_unlock_irqrestore(rdp, flags) {
if (rcu_segcblist_is_offloaded(rdp->cblist)) {
// is offloaded right now so:
raw_spin_unlock_irqrestore(rdp, flags);

And that will explode because that's an impaired unlock on nocb_lock.

\
 
 \ /
  Last update: 2021-07-28 01:09    [W:0.108 / U:0.024 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site