Subject: Re: reproducable panic eviction work queue

From: Nikolay Aleksandrov <[email-masked]>
Date: Wed, 22 Jul 2015 11:11:10 +0200

On 07/22/2015 10:17 AM, Frank Schreuder wrote:
> I got some additional information from syslog:
> 
> Jul 22 09:49:33 dommy0 kernel: [  675.987890] NMI watchdog: BUG: soft lockup - CPU#3 stuck for 22s! [kworker/3:1:42]
> Jul 22 09:49:42 dommy0 kernel: [  685.114033] INFO: rcu_sched self-detected stall on CPU { 3}  (t=39918 jiffies g=988 c=987 q=23168)
> 
> Thanks,
> Frank
> 
> 

Hi,
It looks like it's happening because of the evict_again logic, I think we should also
add Florian's first suggestion about simplifying it to the patch and just skip the
entry if we can't delete its timer otherwise we can restart the eviction and see
entries that already had their timer stopped by us and can keep restarting for
a long time.
Here's an updated patch that removes the evict_again logic.

diff --git a/include/net/inet_frag.h b/include/net/inet_frag.h
index e1300b3dd597..56a3a5685f76 100644
--- a/include/net/inet_frag.h
+++ b/include/net/inet_frag.h
@@ -45,6 +45,7 @@ enum {
  * @flags: fragment queue flags
  * @max_size: maximum received fragment size
  * @net: namespace that this frag belongs to
+ * @list_evictor: list of queues to forcefully evict (e.g. due to low memory)
  */
 struct inet_frag_queue {
 	spinlock_t		lock;
@@ -59,6 +60,7 @@ struct inet_frag_queue {
 	__u8			flags;
 	u16			max_size;
 	struct netns_frags	*net;
+	struct hlist_node	list_evictor;
 };
 
 #define INETFRAGS_HASHSZ	1024
diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 5e346a082e5f..aaae37949c14 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -138,27 +138,17 @@ evict_again:
 		if (!inet_fragq_should_evict(fq))
 			continue;
 
-		if (!del_timer(&fq->timer)) {
-			/* q expiring right now thus increment its refcount so
-			 * it won't be freed under us and wait until the timer
-			 * has finished executing then destroy it
-			 */
-			atomic_inc(&fq->refcnt);
-			spin_unlock(&hb->chain_lock);
-			del_timer_sync(&fq->timer);
-			inet_frag_put(fq, f);
-			goto evict_again;
-		}
+		if (!del_timer(&fq->timer))
+			continue;
 
 		fq->flags |= INET_FRAG_EVICTED;
-		hlist_del(&fq->list);
-		hlist_add_head(&fq->list, &expired);
+		hlist_add_head(&fq->list_evictor, &expired);
 		++evicted;
 	}
 
 	spin_unlock(&hb->chain_lock);
 
-	hlist_for_each_entry_safe(fq, n, &expired, list)
+	hlist_for_each_entry_safe(fq, n, &expired, list_evictor)
 		f->frag_expire((unsigned long) fq);
 
 	return evicted;
@@ -284,8 +274,7 @@ static inline void fq_unlink(struct inet_frag_queue *fq, struct inet_frags *f)
 	struct inet_frag_bucket *hb;
 
 	hb = get_frag_bucket_locked(fq, f);
-	if (!(fq->flags & INET_FRAG_EVICTED))
-		hlist_del(&fq->list);
+	hlist_del(&fq->list);
 	spin_unlock(&hb->chain_lock);
 }