inet: frags: better deal with smp races

Multiple cpus might attempt to insert a new fragment in rhashtable,
if for example RPS is buggy, as reported by 배석진 in
https://patchwork.ozlabs.org/patch/994601/

We use rhashtable_lookup_get_insert_key() instead of
rhashtable_insert_fast() to let cpus losing the race
free their own inet_frag_queue and use the one that
was inserted by another cpu.

Fixes: 648700f76b03 ("inet: frags: use rhashtables for reassembly units")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: 배석진 <soukjin.bae@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 14 deletions

diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index bcb11f3a27c0..760a9e52e02b 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -178,21 +178,22 @@ static struct inet_frag_queue *inet_frag_alloc(struct netns_frags *nf,
 }
 
 static struct inet_frag_queue *inet_frag_create(struct netns_frags *nf,
-                                                void *arg)
+                                                void *arg,
+                                                struct inet_frag_queue **prev)
 {
         struct inet_frags *f = nf->f;
         struct inet_frag_queue *q;
-        int err;
 
         q = inet_frag_alloc(nf, f, arg);
-        if (!q)
+        if (!q) {
+                *prev = ERR_PTR(-ENOMEM);
                 return NULL;
-
+        }
         mod_timer(&q->timer, jiffies + nf->timeout);
 
-        err = rhashtable_insert_fast(&nf->rhashtable, &q->node,
-                                     f->rhash_params);
-        if (err < 0) {
+        *prev = rhashtable_lookup_get_insert_key(&nf->rhashtable, &q->key,
+                                                 &q->node, f->rhash_params);
+        if (*prev) {
                 q->flags |= INET_FRAG_COMPLETE;
                 inet_frag_kill(q);
                 inet_frag_destroy(q);
@@ -204,22 +205,22 @@ static struct inet_frag_queue *inet_frag_create(struct netns_frags *nf,
 /* TODO : call from rcu_read_lock() and no longer use refcount_inc_not_zero() */
 struct inet_frag_queue *inet_frag_find(struct netns_frags *nf, void *key)
 {
-        struct inet_frag_queue *fq;
+        struct inet_frag_queue *fq = NULL, *prev;
 
         if (!nf->high_thresh || frag_mem_limit(nf) > nf->high_thresh)
                 return NULL;
 
         rcu_read_lock();
 
-        fq = rhashtable_lookup(&nf->rhashtable, key, nf->f->rhash_params);
-        if (fq) {
+        prev = rhashtable_lookup(&nf->rhashtable, key, nf->f->rhash_params);
+        if (!prev)
+                fq = inet_frag_create(nf, key, &prev);
+        if (prev && !IS_ERR(prev)) {
+                fq = prev;
                 if (!refcount_inc_not_zero(&fq->refcnt))
                         fq = NULL;
-                rcu_read_unlock();
-                return fq;
         }
         rcu_read_unlock();
-
-        return inet_frag_create(nf, key);
+        return fq;
 }
 EXPORT_SYMBOL(inet_frag_find);