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SubjectRe: data-race in cgroup_get_tree / proc_cgroup_show
On Mon, Aug 22, 2022 at 01:04:58PM -0400, Gabriel Ryan wrote:
> Hi Christian,
>
> We ran a quick test and confirm your suggestion would eliminate the
> data race alert we observed. If the data race is benign (and it
> appears to be), using WRITE_ONCE(cgrp_dfl_visible, true) instead of
> cmpxchg in cgroup_get_tree() would probably also be ok.

I don't see how the data race can lead to anything but would the following
work?

Thanks.

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index ffaccd6373f1e..a90fdba881bdb 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2172,7 +2172,7 @@ static int cgroup_get_tree(struct fs_context *fc)
struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
int ret;

- cgrp_dfl_visible = true;
+ WRITE_ONCE(cgrp_dfl_visible, true);
cgroup_get_live(&cgrp_dfl_root.cgrp);
ctx->root = &cgrp_dfl_root;

@@ -6056,7 +6056,7 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
struct cgroup *cgrp;
int ssid, count = 0;

- if (root == &cgrp_dfl_root && !cgrp_dfl_visible)
+ if (root == &cgrp_dfl_root && !READ_ONCE(cgrp_dfl_visible))
continue;

seq_printf(m, "%d:", root->hierarchy_id);

--
tejun

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