From mboxrd@z Thu Jan 1 00:00:00 1970 From: Aleksa Sarai Subject: [PATCH 1/2] cgroup: get a ref to source csses when migrating Date: Sun, 23 Aug 2015 23:10:31 +1000 Message-ID: <1440335432-4202-2-git-send-email-cyphar@cyphar.com> References: <1440335432-4202-1-git-send-email-cyphar@cyphar.com> Return-path: In-Reply-To: <1440335432-4202-1-git-send-email-cyphar-gVpy/LI/lHzQT0dZR+AlfA@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Aleksa Sarai Grab a ref to each source css being migrated from, otherwise it's possible for the refcount to reach zero between ->can_attach() and ->cancel_attach(). This means that operations on the task's old css (such as container_of(...)) become unsafe, as we may be operating on a different css. Signed-off-by: Aleksa Sarai --- kernel/cgroup.c | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 4ec1b7ee5de8..6cbfbe36284d 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -2305,6 +2305,17 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader, if (list_empty(&tset.src_csets)) return 0; + /* + * Fetch a ref of each css, so that the old task's css doesn't get reaped + * between ->can_attach() and ->cancel_attach(). + */ + down_read(&css_set_rwsem); + list_for_each_entry(cset, &tset.src_csets, mg_node) { + for_each_e_css(css, i, cgrp) + css_get(cset->subsys[i]); + } + up_read(&css_set_rwsem); + /* check that we can legitimately attach to the cgroup */ for_each_e_css(css, i, cgrp) { if (css->ss->can_attach) { @@ -2341,7 +2352,7 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader, css->ss->attach(css, &tset); ret = 0; - goto out_release_tset; + goto out_deref_src; out_cancel_attach: for_each_e_css(css, i, cgrp) { @@ -2350,7 +2361,13 @@ out_cancel_attach: if (css->ss->cancel_attach) css->ss->cancel_attach(css, &tset); } -out_release_tset: +out_deref_src: + down_read(&css_set_rwsem); + list_for_each_entry(cset, &tset.src_csets, mg_node) { + for_each_e_css(css, i, cgrp) + css_put(cset->subsys[i]); + } + up_read(&css_set_rwsem); down_write(&css_set_rwsem); list_splice_init(&tset.dst_csets, &tset.src_csets); list_for_each_entry_safe(cset, tmp_cset, &tset.src_csets, mg_node) { -- 2.5.0