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Commit 32707c4d authored by Herbert Xu's avatar Herbert Xu Committed by David S. Miller
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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: default avatarHerbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 5080e28d

net/ipv4/inet_fragment.c

+10 −6
Original line number Diff line number Diff line
@@ -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);