[patch 02/86] mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction

[Andrew Morton <akpm@linux-foundation.org>]

From: Roman Gushchin <guro@fb.com>
Subject: mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction

Christian reported a warning like the following obtained during running
some KVM-related tests on s390:

WARNING: CPU: 8 PID: 208 at lib/percpu-refcount.c:108 percpu_ref_exit+0x50/0x58
Modules linked in: kvm(-) xt_CHECKSUM xt_MASQUERADE bonding xt_tcpudp ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_na>
CPU: 8 PID: 208 Comm: kworker/8:1 Not tainted 5.2.0+ #66
Hardware name: IBM 2964 NC9 712 (LPAR)
Workqueue: events sysfs_slab_remove_workfn
Krnl PSW : 0704e00180000000 0000001529746850 (percpu_ref_exit+0x50/0x58)
           R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
Krnl GPRS: 00000000ffff8808 0000001529746740 000003f4e30e8e18 0036008100000000
           0000001f00000000 0035008100000000 0000001fb3573ab8 0000000000000000
           0000001fbdb6de00 0000000000000000 0000001529f01328 0000001fb3573b00
           0000001fbb27e000 0000001fbdb69300 000003e009263d00 000003e009263cd0
Krnl Code: 0000001529746842: f0a0000407fe        srp        4(11,%r0),2046,0
           0000001529746848: 47000700            bc         0,1792
          #000000152974684c: a7f40001            brc        15,152974684e
          >0000001529746850: a7f4fff2            brc        15,1529746834
           0000001529746854: 0707                bcr        0,%r7
           0000001529746856: 0707                bcr        0,%r7
           0000001529746858: eb8ff0580024        stmg       %r8,%r15,88(%r15)
           000000152974685e: a738ffff            lhi        %r3,-1
Call Trace:
([<000003e009263d00>] 0x3e009263d00)
 [<00000015293252ea>] slab_kmem_cache_release+0x3a/0x70
 [<0000001529b04882>] kobject_put+0xaa/0xe8
 [<000000152918cf28>] process_one_work+0x1e8/0x428
 [<000000152918d1b0>] worker_thread+0x48/0x460
 [<00000015291942c6>] kthread+0x126/0x160
 [<0000001529b22344>] ret_from_fork+0x28/0x30
 [<0000001529b2234c>] kernel_thread_starter+0x0/0x10
Last Breaking-Event-Address:
 [<000000152974684c>] percpu_ref_exit+0x4c/0x58
---[ end trace b035e7da5788eb09 ]---

The problem occurs because kmem_cache_destroy() is called immediately
after deleting of a memcg, so it races with the memcg kmem_cache
deactivation.

flush_memcg_workqueue() at the beginning of kmem_cache_destroy() is
supposed to guarantee that all deactivation processes are finished, but
failed to do so.  It waits for an rcu grace period, after which all
children kmem_caches should be deactivated.  During the deactivation
percpu_ref_kill() is called for non root kmem_cache refcounters, but it
requires yet another rcu grace period to finish the transition to the
atomic (dead) state.

So in a rare case when not all children kmem_caches are destroyed at the
moment when the root kmem_cache is about to be gone, we need to wait
another rcu grace period before destroying the root kmem_cache.

This issue can be triggered only with dynamically created kmem_caches
which are used with memcg accounting.  In this case per-memcg child
kmem_caches are created.  They are deactivated from the cgroup removing
path.  If the destruction of the root kmem_cache is racing with the
removal of the cgroup (both are quite complicated multi-stage processes),
the described issue can occur.  The only known way to trigger it in the
real life, is to unload some kernel module which creates a dedicated
kmem_cache, used from different memory cgroups with GFP_ACCOUNT flag.  If
the unloading happens immediately after calling rmdir on the corresponding
cgroup, there is some chance to trigger the issue.

Link: http://lkml.kernel.org/r/20191129025011.3076017-1-guro@fb.com
Fixes: f0a3a24b532d ("mm: memcg/slab: rework non-root kmem_cache lifecycle management")
Signed-off-by: Roman Gushchin <guro@fb.com>
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/slab_common.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/mm/slab_common.c~mm-memcg-slab-wait-for-root-kmem_cache-refcnt-killing-on-root-kmem_cache-destruction
+++ a/mm/slab_common.c
@@ -904,6 +904,18 @@ static void flush_memcg_workqueue(struct
 	 * previous workitems on workqueue are processed.
 	 */
 	flush_workqueue(memcg_kmem_cache_wq);
+
+	/*
+	 * If we're racing with children kmem_cache deactivation, it might
+	 * take another rcu grace period to complete their destruction.
+	 * At this moment the corresponding percpu_ref_kill() call should be
+	 * done, but it might take another rcu grace period to complete
+	 * switching to the atomic mode.
+	 * Please, note that we check without grabbing the slab_mutex. It's safe
+	 * because at this moment the children list can't grow.
+	 */
+	if (!list_empty(&s->memcg_params.children))
+		rcu_barrier();
 }
 #else
 static inline int shutdown_memcg_caches(struct kmem_cache *s)
_