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Message-ID: <0b0dc523bf795e78d72c341764a3954f7a7924be.camel@kernel.org>
Date: Mon, 19 Apr 2021 15:41:11 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Yejune Deng <yejune.deng@...il.com>, idryomov@...il.com
Cc: ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
Patrick Donnelly <pdonnell@...hat.com>
Subject: Re: [PATCH] ceph: convert {n}ref from atomic_t to refcount_t
On Wed, 2021-03-24 at 18:26 +0800, Yejune Deng wrote:
> refcount_t type should be used instead of atomic_t when the variable
> is used as a reference counter. This is because the implementation of
> refcount_t can prevent overflows and detect possible use-after-free.
>
> Signed-off-by: Yejune Deng <yejune.deng@...il.com>
> ---
> fs/ceph/mds_client.h | 2 +-
> fs/ceph/snap.c | 27 +++++++++++++++------------
> fs/ceph/super.h | 2 +-
> 3 files changed, 17 insertions(+), 14 deletions(-)
>
> diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
> index eaa7c5422116..bf99c5ba47fc 100644
> --- a/fs/ceph/mds_client.h
> +++ b/fs/ceph/mds_client.h
> @@ -351,7 +351,7 @@ struct ceph_pool_perm {
> struct ceph_snapid_map {
> struct rb_node node;
> struct list_head lru;
> - atomic_t ref;
> + refcount_t ref;
> u64 snap;
> dev_t dev;
> unsigned long last_used;
> diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
> index 0728b01d4d43..c0fbbb56b259 100644
> --- a/fs/ceph/snap.c
> +++ b/fs/ceph/snap.c
> @@ -66,14 +66,15 @@ void ceph_get_snap_realm(struct ceph_mds_client *mdsc,
> struct ceph_snap_realm *realm)
> {
> dout("get_realm %p %d -> %d\n", realm,
> - atomic_read(&realm->nref), atomic_read(&realm->nref)+1);
> + refcount_read(&realm->nref), refcount_read(&realm->nref)+1);
> /*
> * since we _only_ increment realm refs or empty the empty
> * list with snap_rwsem held, adjusting the empty list here is
> * safe. we do need to protect against concurrent empty list
> * additions, however.
> */
> - if (atomic_inc_return(&realm->nref) == 1) {
> + refcount_inc(&realm->nref);
> + if (refcount_read(&realm->nref) == 1) {
> spin_lock(&mdsc->snap_empty_lock);
> list_del_init(&realm->empty_item);
> spin_unlock(&mdsc->snap_empty_lock);
> @@ -117,7 +118,7 @@ static struct ceph_snap_realm *ceph_create_snap_realm(
> if (!realm)
> return ERR_PTR(-ENOMEM);
>
> - atomic_set(&realm->nref, 1); /* for caller */
> + refcount_set(&realm->nref, 1); /* for caller */
> realm->ino = ino;
> INIT_LIST_HEAD(&realm->children);
> INIT_LIST_HEAD(&realm->child_item);
> @@ -199,8 +200,8 @@ static void __put_snap_realm(struct ceph_mds_client *mdsc,
> struct ceph_snap_realm *realm)
> {
> dout("__put_snap_realm %llx %p %d -> %d\n", realm->ino, realm,
> - atomic_read(&realm->nref), atomic_read(&realm->nref)-1);
> - if (atomic_dec_and_test(&realm->nref))
> + refcount_read(&realm->nref), refcount_read(&realm->nref)-1);
> + if (refcount_dec_and_test(&realm->nref))
> __destroy_snap_realm(mdsc, realm);
> }
>
> @@ -211,8 +212,8 @@ void ceph_put_snap_realm(struct ceph_mds_client *mdsc,
> struct ceph_snap_realm *realm)
> {
> dout("put_snap_realm %llx %p %d -> %d\n", realm->ino, realm,
> - atomic_read(&realm->nref), atomic_read(&realm->nref)-1);
> - if (!atomic_dec_and_test(&realm->nref))
> + refcount_read(&realm->nref), refcount_read(&realm->nref)-1);
> + if (!refcount_dec_and_test(&realm->nref))
> return;
>
> if (down_write_trylock(&mdsc->snap_rwsem)) {
> @@ -1034,7 +1035,8 @@ struct ceph_snapid_map* ceph_get_snapid_map(struct ceph_mds_client *mdsc,
> } else if (snap < exist->snap) {
> p = &(*p)->rb_right;
> } else {
> - if (atomic_inc_return(&exist->ref) == 1)
> + refcount_inc(&exist->ref);
> + if (refcount_read(&exist->ref) == 1)
> list_del_init(&exist->lru);
> break;
> }
> @@ -1057,7 +1059,7 @@ struct ceph_snapid_map* ceph_get_snapid_map(struct ceph_mds_client *mdsc,
> }
>
> INIT_LIST_HEAD(&sm->lru);
> - atomic_set(&sm->ref, 1);
> + refcount_set(&sm->ref, 1);
> sm->snap = snap;
>
> exist = NULL;
> @@ -1076,7 +1078,8 @@ struct ceph_snapid_map* ceph_get_snapid_map(struct ceph_mds_client *mdsc,
> exist = NULL;
> }
> if (exist) {
> - if (atomic_inc_return(&exist->ref) == 1)
> + refcount_inc(&exist->ref);
> + if (refcount_read(&exist->ref) == 1)
Hi Yejune,
I believe this patch is causing the regression reported here:
https://tracker.ceph.com/issues/50281
Note that the above two operations together aren't atomic like
atomic_inc_return is, and I suspect this is causing the object to remain
on the LRU list after its refcount has made a 0->1 transition. Does
refcount_t allow a 0->1 transition like this code does?
In any case, I'm dropping this patch for now.
> list_del_init(&exist->lru);
> } else {
> rb_link_node(&sm->node, parent, p);
> @@ -1099,7 +1102,7 @@ void ceph_put_snapid_map(struct ceph_mds_client* mdsc,
> {
> if (!sm)
> return;
> - if (atomic_dec_and_lock(&sm->ref, &mdsc->snapid_map_lock)) {
> + if (refcount_dec_and_lock(&sm->ref, &mdsc->snapid_map_lock)) {
> if (!RB_EMPTY_NODE(&sm->node)) {
> sm->last_used = jiffies;
> list_add_tail(&sm->lru, &mdsc->snapid_map_lru);
> @@ -1161,7 +1164,7 @@ void ceph_cleanup_snapid_map(struct ceph_mds_client *mdsc)
> sm = list_first_entry(&to_free, struct ceph_snapid_map, lru);
> list_del(&sm->lru);
> free_anon_bdev(sm->dev);
> - if (WARN_ON_ONCE(atomic_read(&sm->ref))) {
> + if (WARN_ON_ONCE(refcount_read(&sm->ref))) {
> pr_err("snapid map %llx -> %x still in use\n",
> sm->snap, sm->dev);
> }
> diff --git a/fs/ceph/super.h b/fs/ceph/super.h
> index c48bb30c8d70..062123a73ef1 100644
> --- a/fs/ceph/super.h
> +++ b/fs/ceph/super.h
> @@ -835,7 +835,7 @@ struct ceph_readdir_cache_control {
> struct ceph_snap_realm {
> u64 ino;
> struct inode *inode;
> - atomic_t nref;
> + refcount_t nref;
> struct rb_node node;
>
> u64 created, seq;
Thanks,
--
Jeff Layton <jlayton@...nel.org>
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