Jeff Layton <jlayton@xxxxxxxxxx> writes: > 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 > Cc: Sage Weil <sage@xxxxxxxxxx> > Reported-by: Mark Nelson <mnelson@xxxxxxxxxx> > URL: https://tracker.ceph.com/issues/46419 > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > --- > fs/ceph/snap.c | 34 +++++++++++++++++----------------- > 1 file changed, 17 insertions(+), 17 deletions(-) > > v2: No functional changes, but I cleaned up the comments a bit and > added another in __put_snap_realm. > > diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c > index 9dbc92cfda38..158c11e96fb7 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_add_unless(&realm->nref, 1, 0)) Here you could probably use atomic_inc_not_zero() instead. But other than that it looks good. Thanks a lot for solving yet another locking puzzle! Reviewed-by: Luis Henriques <lhenriques@xxxxxxx> Cheers, -- Luis > + 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); > } > -- > > 2.31.1 >
