From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 670EAC4338F for ; Sun, 15 Aug 2021 13:50:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3E5F060FC3 for ; Sun, 15 Aug 2021 13:50:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233252AbhHONvU (ORCPT ); Sun, 15 Aug 2021 09:51:20 -0400 Received: from mail.kernel.org ([198.145.29.99]:34456 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231881AbhHONvT (ORCPT ); Sun, 15 Aug 2021 09:51:19 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 3C27A60EB5; Sun, 15 Aug 2021 13:50:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1629035449; bh=H/c7EzkdVLKtQIiUkbVhOkcryCB79SOj6w4PKhpUXcg=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=L78uxsleke8jIPJ1ik/I6yenpRmSpAMuQhkseJki2T2tuTY7RDcLPMBBDnWq+wrM+ kopuL3wYxwU1ZScfC/6FRf6BY2lH+jS16ggr5gcnFRyYQULV/bB6XkzJS65+z9/eSv p1e7INkpQU1x96IRA1DtdigemnOWly0muERWKrMSC9+NBbVx+BVxHyNO8p28YS6Cv8 6KvYRiIGnGiV0kzJ7rA+Ge5/1h0uvkDAuvHmDaTCIbxcH/UvXAC5cSiaf1uAK6fnmk M8LwQFSYjszRwCCZQC8HVV5zMwNgFUYMbB8SC16q+PXr7tK8KYSUaElaXCaG1UATHN L6kBSEug3cibQ== Message-ID: <97e1f7dfb46c148144eca610c7b4cc50de948fcf.camel@kernel.org> Subject: Re: FAILED: patch "[PATCH] ceph: take snap_empty_lock atomically with snaprealm refcount" failed to apply to 5.13-stable tree From: Jeff Layton To: gregkh@linuxfoundation.org, idryomov@gmail.com, lhenriques@suse.de, mnelson@redhat.com Cc: stable@vger.kernel.org Date: Sun, 15 Aug 2021 09:50:48 -0400 In-Reply-To: <162903121195232@kroah.com> References: <162903121195232@kroah.com> Content-Type: text/plain; charset="ISO-8859-15" User-Agent: Evolution 3.40.3 (3.40.3-1.fc34) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Sun, 2021-08-15 at 14:40 +0200, gregkh@linuxfoundation.org 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 . > > thanks, > > greg k-h > > ------------------ original commit in Linus's tree ------------------ > > From 8434ffe71c874b9c4e184b88d25de98c2bf5fe3f Mon Sep 17 00:00:00 2001 > From: Jeff Layton > 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@vger.kernel.org > URL: https://tracker.ceph.com/issues/46419 > Reported-by: Mark Nelson > Signed-off-by: Jeff Layton > Reviewed-by: Luis Henriques > Signed-off-by: Ilya Dryomov > > 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? -- Jeff Layton