On Tue, May 28, 2019 at 06:31:00AM -0700, Eric Dumazet wrote:
>
> This smp_store_release() is a left over of the first version of the patch,
> where
> there was no rcu grace period enforcement.
>
> I do not believe there is harm letting this, but if you disagree
> please send a patch ;)
I see now that it is actually relying on the barrier/locking
semantics of call_rcu vs. rcu_read_lock. So the smp_store_release
and READ_ONCE are simply unnecessary and could be confusing to
future readers.
---8<---
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 <herb...@gondor.apana.org.au>
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);
--
Email: Herbert Xu <herb...@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
