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>

diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 2b816f1ebbb4ad6fc65aaeff7fadcea8a78e5f9f..35e9784fab4e1098ddfd52fffc630e31b46e1502 100644 (file)
--- 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);