On Sun, Aug 15, 2021 at 09:50:48AM -0400, Jeff Layton wrote: > On Sun, 2021-08-15 at 14:40 +0200, gregkh@xxxxxxxxxxxxxxxxxxx wrote: > > The patch below does not apply to the 5.13-stable tree. > > If someone wants it applied there, or to any other stable or longterm > > tree, then please email the backport, including the original git commit > > id to <stable@xxxxxxxxxxxxxxx>. > > > > thanks, > > > > greg k-h > > > > ------------------ original commit in Linus's tree ------------------ > > > > From 8434ffe71c874b9c4e184b88d25de98c2bf5fe3f Mon Sep 17 00:00:00 2001 > > From: Jeff Layton <jlayton@xxxxxxxxxx> > > Date: Tue, 3 Aug 2021 12:47:34 -0400 > > Subject: [PATCH] ceph: take snap_empty_lock atomically with snaprealm refcount > > change > > > > There is a race in ceph_put_snap_realm. The change to the nref and the > > spinlock acquisition are not done atomically, so you could decrement > > nref, and before you take the spinlock, the nref is incremented again. > > At that point, you end up putting it on the empty list when it > > shouldn't be there. Eventually __cleanup_empty_realms runs and frees > > it when it's still in-use. > > > > Fix this by protecting the 1->0 transition with atomic_dec_and_lock, > > and just drop the spinlock if we can get the rwsem. > > > > Because these objects can also undergo a 0->1 refcount transition, we > > must protect that change as well with the spinlock. Increment locklessly > > unless the value is at 0, in which case we take the spinlock, increment > > and then take it off the empty list if it did the 0->1 transition. > > > > With these changes, I'm removing the dout() messages from these > > functions, as well as in __put_snap_realm. They've always been racy, and > > it's better to not print values that may be misleading. > > > > Cc: stable@xxxxxxxxxxxxxxx > > URL: https://tracker.ceph.com/issues/46419 > > Reported-by: Mark Nelson <mnelson@xxxxxxxxxx> > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > > Reviewed-by: Luis Henriques <lhenriques@xxxxxxx> > > Signed-off-by: Ilya Dryomov <idryomov@xxxxxxxxx> > > > > diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c > > index 4ac0606dcbd4..4c6bd1042c94 100644 > > --- a/fs/ceph/snap.c > > +++ b/fs/ceph/snap.c > > @@ -67,19 +67,19 @@ void ceph_get_snap_realm(struct ceph_mds_client *mdsc, > > { > > lockdep_assert_held(&mdsc->snap_rwsem); > > > > - dout("get_realm %p %d -> %d\n", realm, > > - atomic_read(&realm->nref), atomic_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. > > + * The 0->1 and 1->0 transitions must take the snap_empty_lock > > + * atomically with the refcount change. Go ahead and bump the > > + * nref here, unless it's 0, in which case we take the spinlock > > + * and then do the increment and remove it from the list. > > */ > > - if (atomic_inc_return(&realm->nref) == 1) { > > - spin_lock(&mdsc->snap_empty_lock); > > + if (atomic_inc_not_zero(&realm->nref)) > > + return; > > + > > + spin_lock(&mdsc->snap_empty_lock); > > + if (atomic_inc_return(&realm->nref) == 1) > > list_del_init(&realm->empty_item); > > - spin_unlock(&mdsc->snap_empty_lock); > > - } > > + spin_unlock(&mdsc->snap_empty_lock); > > } > > > > static void __insert_snap_realm(struct rb_root *root, > > @@ -208,28 +208,28 @@ static void __put_snap_realm(struct ceph_mds_client *mdsc, > > { > > lockdep_assert_held_write(&mdsc->snap_rwsem); > > > > - dout("__put_snap_realm %llx %p %d -> %d\n", realm->ino, realm, > > - atomic_read(&realm->nref), atomic_read(&realm->nref)-1); > > + /* > > + * We do not require the snap_empty_lock here, as any caller that > > + * increments the value must hold the snap_rwsem. > > + */ > > if (atomic_dec_and_test(&realm->nref)) > > __destroy_snap_realm(mdsc, realm); > > } > > > > /* > > - * caller needn't hold any locks > > + * See comments in ceph_get_snap_realm. Caller needn't hold any locks. > > */ > > 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)) > > + if (!atomic_dec_and_lock(&realm->nref, &mdsc->snap_empty_lock)) > > return; > > > > if (down_write_trylock(&mdsc->snap_rwsem)) { > > + spin_unlock(&mdsc->snap_empty_lock); > > __destroy_snap_realm(mdsc, realm); > > up_write(&mdsc->snap_rwsem); > > } else { > > - spin_lock(&mdsc->snap_empty_lock); > > list_add(&realm->empty_item, &mdsc->snap_empty); > > spin_unlock(&mdsc->snap_empty_lock); > > } > > > > Ahh, I forgot to account for some new lockdep annotation when I marked > these for stable. I think what we should probably do here is cherry-pick > these as prerequisites before applying: > > a6862e6708c1 ceph: add some lockdep assertions around snaprealm handling > df2c0cb7f8e8 ceph: clean up locking annotation for ceph_get_snap_realm and __lookup_snap_realm > > The first one should fix up the merge conflict, and the second will fix > up a couple of bogus lockdep warnings that pop up from a6862e6708c1. > > Greg, does that sound OK? Yup, that worked, all now queued up, thanks! greg k-h
