Re: [PATCH-next v3] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT

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On 12/14/21 23:36, Davidlohr Bueso wrote:
On Tue, 14 Dec 2021, Waiman Long wrote:

@@ -2189,7 +2194,7 @@ static void drain_local_stock(struct work_struct *dummy)
     * drain_stock races is that we always operate on local CPU stock
     * here with IRQ disabled
     */
-    local_irq_save(flags);
+    local_lock_irqsave(&memcg_stock.lock, flags);

    stock = this_cpu_ptr(&memcg_stock);
    drain_obj_stock(&stock->irq_obj);

So here there is still the problem that you can end up taking sleeping locks
with irqs disabled via obj_cgroup_put() >> obj_cgroup_release() - ie: the
percpu_ref_switch_lock and css_set_lock. It had occurred to me to promote
the former to a raw spinlock, but doubt we can get away with the latter.

Yes, it is tricky to avoid all these. One possibility to to delay obj_cgroup_release() through RCU. However, this is outside the scope of this patch.

Cheers,
Longman






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