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authorHerbert Xu <herbert@gondor.apana.org.au>2019-05-29 13:40:26 +0800
committerDavid S. Miller <davem@davemloft.net>2019-05-30 11:51:29 -0700
commit32707c4dfa20b01d6a3c8d2797daa52bfc188add (patch)
tree14d57d834cd97a5dd58b8ef0cb44847f83fb45ac /net/ipv4/inet_fragment.c
parent5080e28d94a27e182e323854a9ba2a0718e9e3bf (diff)
inet: frags: Remove unnecessary smp_store_release/READ_ONCE
The smp_store_release call in fqdir_exit cannot protect the setting of fqdir->dead as claimed because its memory barrier is only guaranteed to be one-way and the barrier precedes the setting of fqdir->dead. IOW it doesn't provide any barriers between fq->dir and the following hash table destruction. In fact, the code is safe anyway because call_rcu does provide both the memory barrier as well as a guarantee that when the destruction work starts executing all RCU readers will see the updated value for fqdir->dead. Therefore this patch removes the unnecessary smp_store_release call as well as the corresponding READ_ONCE on the read-side in order to not confuse future readers of this code. Comments have been added in their places. Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au> Reviewed-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/ipv4/inet_fragment.c')
-rw-r--r--net/ipv4/inet_fragment.c16
1 files changed, 10 insertions, 6 deletions
diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 2b816f1ebbb4..35e9784fab4e 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -193,10 +193,12 @@ void fqdir_exit(struct fqdir *fqdir)
{
fqdir->high_thresh = 0; /* prevent creation of new frags */
- /* paired with READ_ONCE() in inet_frag_kill() :
- * We want to prevent rhashtable_remove_fast() calls
+ fqdir->dead = true;
+
+ /* call_rcu is supposed to provide memory barrier semantics,
+ * separating the setting of fqdir->dead with the destruction
+ * work. This implicit barrier is paired with inet_frag_kill().
*/
- smp_store_release(&fqdir->dead, true);
INIT_RCU_WORK(&fqdir->destroy_rwork, fqdir_rwork_fn);
queue_rcu_work(system_wq, &fqdir->destroy_rwork);
@@ -214,10 +216,12 @@ void inet_frag_kill(struct inet_frag_queue *fq)
fq->flags |= INET_FRAG_COMPLETE;
rcu_read_lock();
- /* This READ_ONCE() is paired with smp_store_release()
- * in inet_frags_exit_net().
+ /* The RCU read lock provides a memory barrier
+ * guaranteeing that if fqdir->dead is false then
+ * the hash table destruction will not start until
+ * after we unlock. Paired with inet_frags_exit_net().
*/
- if (!READ_ONCE(fqdir->dead)) {
+ if (!fqdir->dead) {
rhashtable_remove_fast(&fqdir->rhashtable, &fq->node,
fqdir->f->rhash_params);
refcount_dec(&fq->refcnt);