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Subject[PATCH RT 5/8] mm: slub: Always flush the delayed empty slubs in flush_all()
5.4.44-rt27-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Kevin Hao <haokexin@gmail.com>

After commit f0b231101c94 ("mm/SLUB: delay giving back empty slubs to
IRQ enabled regions"), when the free_slab() is invoked with the IRQ
disabled, the empty slubs are moved to a per-CPU list and will be
freed after IRQ enabled later. But in the current codes, there is
a check to see if there really has the cpu slub on a specific cpu
before flushing the delayed empty slubs, this may cause a reference
of already released kmem_cache in a scenario like below:
cpu 0 cpu 1
kmem_cache_destroy()
flush_all()
--->IPI flush_cpu_slab()
flush_slab()
deactivate_slab()
discard_slab()
free_slab()
c->page = NULL;
for_each_online_cpu(cpu)
if (!has_cpu_slab(1, s))
continue
this skip to flush the delayed
empty slub released by cpu1
kmem_cache_free(kmem_cache, s)

kmalloc()
__slab_alloc()
free_delayed()
__free_slab()
reference to released kmem_cache

Fixes: f0b231101c94 ("mm/SLUB: delay giving back empty slubs to IRQ enabled regions")
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable-rt@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
mm/slub.c | 3 ---
1 file changed, 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index bea18bbae247..b2809a9ca8f8 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2382,9 +2382,6 @@ static void flush_all(struct kmem_cache *s)
for_each_online_cpu(cpu) {
struct slub_free_list *f;

- if (!has_cpu_slab(cpu, s))
- continue;
-
f = &per_cpu(slub_free_list, cpu);
raw_spin_lock_irq(&f->lock);
list_splice_init(&f->list, &tofree);
--
2.26.2

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