From 32707c4dfa20b01d6a3c8d2797daa52bfc188add Mon Sep 17 00:00:00 2001 From: Herbert Xu Date: Wed, 29 May 2019 13:40:26 +0800 Subject: [PATCH] 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 Reviewed-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/inet_fragment.c | 16 ++++++++++------ 1 file 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); -- 2.30.2