[PATCH RFC] mm: slub: fix flush_cpu_slab()/__free_slab() invocations in task context.

[Maurizio Lombardi <mlombard@xxxxxxxxxx>]

Commit 5a836bf6b09f ("mm: slub: move flush_cpu_slab() invocations
__free_slab() invocations out of IRQ context") moved all flush_cpu_slab()
invocations to the global workqueue to avoid a problem related
with deactivate_slab()/__free_slab() being called from an IRQ context
on PREEMPT_RT kernels.

When the flush_all_cpu_locked() function is called from a task context
the flush_cpu_slab() function should be called directly, without deferring
it to the global workqueue, otherwise a workqueue with the WQ_MEM_RECLAIM
bit set could end up flushing a workqueue without the aforementioned bit,
triggering a dependency issue.

 workqueue: WQ_MEM_RECLAIM nvme-delete-wq:nvme_delete_ctrl_work [nvme_core]
   is flushing !WQ_MEM_RECLAIM events:flush_cpu_slab
 WARNING: CPU: 37 PID: 410 at kernel/workqueue.c:2637
   check_flush_dependency+0x10a/0x120
 Workqueue: nvme-delete-wq nvme_delete_ctrl_work [nvme_core]
 RIP: 0010:check_flush_dependency+0x10a/0x120[  453.262125] Call Trace:
 __flush_work.isra.0+0xbf/0x220
 ? __queue_work+0x1dc/0x420
 flush_all_cpus_locked+0xfb/0x120
 __kmem_cache_shutdown+0x2b/0x320
 kmem_cache_destroy+0x49/0x100
 bioset_exit+0x143/0x190
 blk_release_queue+0xb9/0x100
 kobject_cleanup+0x37/0x130
 nvme_fc_ctrl_free+0xc6/0x150 [nvme_fc]
 nvme_free_ctrl+0x1ac/0x2b0 [nvme_core]

Fixes: 5a836bf6b09f ("mm: slub: move flush_cpu_slab()
invocations __free_slab() invocations out of IRQ context")

Signed-off-by: Maurizio Lombardi <mlombard@xxxxxxxxxx>
---
 mm/slub.c | 34 ++++++++++++++++++++++------------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 862dbd9af4f5..d46ee90651d2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2681,30 +2681,34 @@ struct slub_flush_work {
 	bool skip;
 };
 
+static void flush_cpu_slab(void *d)
+{
+	struct kmem_cache *s = d;
+	struct kmem_cache_cpu *c = this_cpu_ptr(s->cpu_slab);
+
+	if (c->slab)
+		flush_slab(s, c);
+
+	unfreeze_partials(s);
+}
+
 /*
  * Flush cpu slab.
  *
  * Called from CPU work handler with migration disabled.
  */
-static void flush_cpu_slab(struct work_struct *w)
+static void flush_cpu_slab_work(struct work_struct *w)
 {
-	struct kmem_cache *s;
-	struct kmem_cache_cpu *c;
 	struct slub_flush_work *sfw;
 
 	sfw = container_of(w, struct slub_flush_work, work);
 
-	s = sfw->s;
-	c = this_cpu_ptr(s->cpu_slab);
-
-	if (c->slab)
-		flush_slab(s, c);
-
-	unfreeze_partials(s);
+	flush_cpu_slab(sfw->s);
 }
 
-static bool has_cpu_slab(int cpu, struct kmem_cache *s)
+static bool has_cpu_slab(int cpu, void *info)
 {
+	struct kmem_cache *s = info;
 	struct kmem_cache_cpu *c = per_cpu_ptr(s->cpu_slab, cpu);
 
 	return c->slab || slub_percpu_partial(c);
@@ -2721,13 +2725,18 @@ static void flush_all_cpus_locked(struct kmem_cache *s)
 	lockdep_assert_cpus_held();
 	mutex_lock(&flush_lock);
 
+	if (in_task()) {
+		on_each_cpu_cond(has_cpu_slab, flush_cpu_slab, s, 1);
+		goto unlock_exit;
+	}
+
 	for_each_online_cpu(cpu) {
 		sfw = &per_cpu(slub_flush, cpu);
 		if (!has_cpu_slab(cpu, s)) {
 			sfw->skip = true;
 			continue;
 		}
-		INIT_WORK(&sfw->work, flush_cpu_slab);
+		INIT_WORK(&sfw->work, flush_cpu_slab_work);
 		sfw->skip = false;
 		sfw->s = s;
 		schedule_work_on(cpu, &sfw->work);
@@ -2740,6 +2749,7 @@ static void flush_all_cpus_locked(struct kmem_cache *s)
 		flush_work(&sfw->work);
 	}
 
+unlock_exit:
 	mutex_unlock(&flush_lock);
 }
 
-- 
2.31.1