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Date: Sun, 23 Aug 2015 23:10:31 +1000 From: Aleksa Sarai <cyphar@...har.com> To: tj@...nel.org Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, Aleksa Sarai <cyphar@...har.com> Subject: [PATCH 1/2] cgroup: get a ref to source csses when migrating 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 <cyphar@...har.com> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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