2019-10-16 22:22:06

by Aleksa Sarai

Subject: [PATCH v2] cgroup: pids: use atomic64_t for pids->limit

Because pids->limit can be changed concurrently (but we don't want to
take a lock because it would be needlessly expensive), use atomic64_ts
instead.

Fixes: commit 49b786ea146f ("cgroup: implement the PIDs subsystem")
Cc: [email protected] # v4.3+
Signed-off-by: Aleksa Sarai <[email protected]>
---
v2:
* Switch to atomic64_t instead of using {READ,WRITE}_ONCE().
v1: <https://lore.kernel.org/lkml/[email protected]/>

kernel/cgroup/pids.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
index 8e513a573fe9..138059eb730d 100644
--- a/kernel/cgroup/pids.c
+++ b/kernel/cgroup/pids.c
@@ -45,7 +45,7 @@ struct pids_cgroup {
* %PIDS_MAX = (%PID_MAX_LIMIT + 1).
*/
atomic64_t counter;
- int64_t limit;
+ atomic64_t limit;

/* Handle for "pids.events" */
struct cgroup_file events_file;
@@ -73,8 +73,8 @@ pids_css_alloc(struct cgroup_subsys_state *parent)
if (!pids)
return ERR_PTR(-ENOMEM);

- pids->limit = PIDS_MAX;
atomic64_set(&pids->counter, 0);
+ atomic64_set(&pids->limit, PIDS_MAX);
atomic64_set(&pids->events_limit, 0);
return &pids->css;
}
@@ -146,13 +146,14 @@ static int pids_try_charge(struct pids_cgroup *pids, int num)

for (p = pids; parent_pids(p); p = parent_pids(p)) {
int64_t new = atomic64_add_return(num, &p->counter);
+ int64_t limit = atomic64_read(&p->limit);

/*
* Since new is capped to the maximum number of pid_t, if
* p->limit is %PIDS_MAX then we know that this test will never
* fail.
*/
- if (new > p->limit)
+ if (new > limit)
goto revert;
}

@@ -277,7 +278,7 @@ static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
* Limit updates don't need to be mutex'd, since it isn't
* critical that any racing fork()s follow the new limit.
*/
- pids->limit = limit;
+ atomic64_set(&pids->limit, limit);
return nbytes;
}

@@ -285,7 +286,7 @@ static int pids_max_show(struct seq_file *sf, void *v)
{
struct cgroup_subsys_state *css = seq_css(sf);
struct pids_cgroup *pids = css_pids(css);
- int64_t limit = pids->limit;
+ int64_t limit = atomic64_read(&pids->limit);

if (limit >= PIDS_MAX)
seq_printf(sf, "%s\n", PIDS_MAX_STR);
--
2.23.0

2019-10-25 18:56:36

by Tejun Heo

Subject: Re: [PATCH v2] cgroup: pids: use atomic64_t for pids->limit

On Thu, Oct 17, 2019 at 02:50:01AM +1100, Aleksa Sarai wrote:
> Because pids->limit can be changed concurrently (but we don't want to
> take a lock because it would be needlessly expensive), use atomic64_ts
> instead.
>
> Fixes: commit 49b786ea146f ("cgroup: implement the PIDs subsystem")
> Cc: [email protected] # v4.3+
> Signed-off-by: Aleksa Sarai <[email protected]>

Applied to cgroup/for-5.5.

Thanks.

--
tejun