[PATCH] slub: make sysfs file removal asynchronous
From: Tejun Heo
Date: Tue Jun 20 2017 - 16:45:20 EST
bf5eb3de3847 ("slub: separate out sysfs_slab_release() from
sysfs_slab_remove()") made slub sysfs file removals synchronous to
kmem_cache shutdown. Unfortunately, this created a possible ABBA
deadlock between slab_mutex and sysfs draining mechanism triggering
the following lockdep warning.
======================================================
[ INFO: possible circular locking dependency detected ]
4.10.0-test+ #48 Not tainted
-------------------------------------------------------
rmmod/1211 is trying to acquire lock:
(s_active#120){++++.+}, at: [<ffffffff81308073>] kernfs_remove+0x23/0x40
but task is already holding lock:
(slab_mutex){+.+.+.}, at: [<ffffffff8120f691>] kmem_cache_destroy+0x41/0x2d0
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (slab_mutex){+.+.+.}:
lock_acquire+0xf6/0x1f0
__mutex_lock+0x75/0x950
mutex_lock_nested+0x1b/0x20
slab_attr_store+0x75/0xd0
sysfs_kf_write+0x45/0x60
kernfs_fop_write+0x13c/0x1c0
__vfs_write+0x28/0x120
vfs_write+0xc8/0x1e0
SyS_write+0x49/0xa0
entry_SYSCALL_64_fastpath+0x1f/0xc2
-> #0 (s_active#120){++++.+}:
__lock_acquire+0x10ed/0x1260
lock_acquire+0xf6/0x1f0
__kernfs_remove+0x254/0x320
kernfs_remove+0x23/0x40
sysfs_remove_dir+0x51/0x80
kobject_del+0x18/0x50
__kmem_cache_shutdown+0x3e6/0x460
kmem_cache_destroy+0x1fb/0x2d0
kvm_exit+0x2d/0x80 [kvm]
vmx_exit+0x19/0xa1b [kvm_intel]
SyS_delete_module+0x198/0x1f0
entry_SYSCALL_64_fastpath+0x1f/0xc2
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(slab_mutex);
lock(s_active#120);
lock(slab_mutex);
lock(s_active#120);
*** DEADLOCK ***
2 locks held by rmmod/1211:
#0: (cpu_hotplug.dep_map){++++++}, at: [<ffffffff810a7877>] get_online_cpus+0x37/0x80
#1: (slab_mutex){+.+.+.}, at: [<ffffffff8120f691>] kmem_cache_destroy+0x41/0x2d0
stack backtrace:
CPU: 3 PID: 1211 Comm: rmmod Not tainted 4.10.0-test+ #48
Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v02.05 05/07/2012
Call Trace:
dump_stack+0x86/0xc3
print_circular_bug+0x1be/0x210
__lock_acquire+0x10ed/0x1260
? 0xffffffffa0000077
lock_acquire+0xf6/0x1f0
? kernfs_remove+0x23/0x40
__kernfs_remove+0x254/0x320
? kernfs_remove+0x23/0x40
? __kernfs_remove+0x5/0x320
kernfs_remove+0x23/0x40
sysfs_remove_dir+0x51/0x80
kobject_del+0x18/0x50
__kmem_cache_shutdown+0x3e6/0x460
kmem_cache_destroy+0x1fb/0x2d0
kvm_exit+0x2d/0x80 [kvm]
vmx_exit+0x19/0xa1b [kvm_intel]
SyS_delete_module+0x198/0x1f0
? SyS_delete_module+0x5/0x1f0
entry_SYSCALL_64_fastpath+0x1f/0xc2
RIP: 0033:0x7fdf0439f487
RSP: 002b:00007ffcd39eb6b8 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0
RAX: ffffffffffffffda RBX: 0000000000000046 RCX: 00007fdf0439f487
RDX: 000000000000000a RSI: 0000000000000800 RDI: 000055e4e32f3258
RBP: 0000000000000000 R08: 000000000000000a R09: 0000000000000000
R10: 00007fdf0440ace0 R11: 0000000000000206 R12: 000055e4e32f31f0
R13: 00007ffcd39ea6a0 R14: 0000000000000000 R15: 000055e4e32f31f0
It'd be the cleanest to deal with the issue by removing sysfs files
without holding slab_mutex before the rest of shutdown; however, given
the current code structure, it is pretty difficult to do so. This
patch punts sysfs file removal to a work item. Before bf5eb3de3847,
the removal was punted to a RCU delayed work item which is executed
after release. Now, we're punting to a different work item on
shutdown which still maintains the goal removing the sysfs files
earlier when destroying kmem_caches.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Tested-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Fixes: bf5eb3de3847 ("slub: separate out sysfs_slab_release() from sysfs_slab_remove()")
---
include/linux/slub_def.h | 1 +
mm/slub.c | 40 ++++++++++++++++++++++++++--------------
2 files changed, 27 insertions(+), 14 deletions(-)
diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index 07ef550c6627..93315d6b21a8 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -84,6 +84,7 @@ struct kmem_cache {
int red_left_pad; /* Left redzone padding size */
#ifdef CONFIG_SYSFS
struct kobject kobj; /* For sysfs */
+ struct work_struct kobj_remove_work;
#endif
#ifdef CONFIG_MEMCG
struct memcg_cache_params memcg_params;
diff --git a/mm/slub.c b/mm/slub.c
index 7449593fca72..8addc535bcdc 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5625,6 +5625,28 @@ static char *create_unique_id(struct kmem_cache *s)
return name;
}
+static void sysfs_slab_remove_workfn(struct work_struct *work)
+{
+ struct kmem_cache *s =
+ container_of(work, struct kmem_cache, kobj_remove_work);
+
+ if (!s->kobj.state_in_sysfs)
+ /*
+ * For a memcg cache, this may be called during
+ * deactivation and again on shutdown. Remove only once.
+ * A cache is never shut down before deactivation is
+ * complete, so no need to worry about synchronization.
+ */
+ return;
+
+#ifdef CONFIG_MEMCG
+ kset_unregister(s->memcg_kset);
+#endif
+ kobject_uevent(&s->kobj, KOBJ_REMOVE);
+ kobject_del(&s->kobj);
+ kobject_put(&s->kobj);
+}
+
static int sysfs_slab_add(struct kmem_cache *s)
{
int err;
@@ -5632,6 +5654,8 @@ static int sysfs_slab_add(struct kmem_cache *s)
struct kset *kset = cache_kset(s);
int unmergeable = slab_unmergeable(s);
+ INIT_WORK(&s->kobj_remove_work, sysfs_slab_remove_workfn);
+
if (!kset) {
kobject_init(&s->kobj, &slab_ktype);
return 0;
@@ -5695,20 +5719,8 @@ static void sysfs_slab_remove(struct kmem_cache *s)
*/
return;
- if (!s->kobj.state_in_sysfs)
- /*
- * For a memcg cache, this may be called during
- * deactivation and again on shutdown. Remove only once.
- * A cache is never shut down before deactivation is
- * complete, so no need to worry about synchronization.
- */
- return;
-
-#ifdef CONFIG_MEMCG
- kset_unregister(s->memcg_kset);
-#endif
- kobject_uevent(&s->kobj, KOBJ_REMOVE);
- kobject_del(&s->kobj);
+ kobject_get(&s->kobj);
+ schedule_work(&s->kobj_remove_work);
}
void sysfs_slab_release(struct kmem_cache *s)