Re: [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability
From: Frederic Weisbecker <frederic@kernel.org>
Date: 2021-07-27 23:08:19
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linux-rt-users, lkml
On Wed, Jul 21, 2021 at 12:51:17PM +0100, Valentin Schneider wrote:
quoted hunk ↗ jump to hunk
Signed-off-by: Valentin Schneider <redacted> --- 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.
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