[PATCH RT 2/4] mm: slub: Always flush the delayed empty slubs in flush_all()

[zanussi@kernel.org]

From: Kevin Hao <haokexin@gmail.com>

v4.19.127-rt55-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


[ Upstream commit 23a2c31b19e99beaf5107071b0f32a596202cdae ]

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: Tom Zanussi <zanussi@kernel.org>
---
 mm/slub.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index d243c6ef7fc9..a9473bbb1338 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2341,9 +2341,6 @@ 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);
-- 
2.17.1