Re: [patch] mm, oom: remove task_lock protecting comm printing

Michal Hocko
2015-09-23T07:44:25Z

On Tue 22-09-15 16:30:13, David Rientjes wrote:
> The oom killer takes task_lock() in a couple of places solely to protect
> printing the task's comm.
>
> A process's comm, including current's comm, may change due to
> /proc/pid/comm or PR_SET_NAME.
>
> The comm will always be NULL-terminated, so the worst race scenario would
> only be during update. We can tolerate a comm being printed that is in
> the middle of an update to avoid taking the lock.
>
> Other locations in the kernel have already dropped task_lock() when
> printing comm, so this is consistent.

cpuset_print_task_mems_allowed seems unrelated and it would probably
deserve a separate pach.

> Suggested-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: David Rientjes <rientjes@google.com>

For the task_lock part
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> include/linux/cpuset.h | 4 ++--
> kernel/cpuset.c | 14 +++++++-------
> mm/oom_kill.c | 8 +-------
> 3 files changed, 10 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -93,7 +93,7 @@ extern int current_cpuset_is_being_rebound(void);
>
> extern void rebuild_sched_domains(void);
>
> -extern void cpuset_print_task_mems_allowed(struct task_struct *p);
> +extern void cpuset_print_current_mems_allowed(void);
>
> /*
> * read_mems_allowed_begin is required when making decisions involving
> @@ -219,7 +219,7 @@ static inline void rebuild_sched_domains(void)
> partition_sched_domains(1, NULL, NULL);
> }
>
> -static inline void cpuset_print_task_mems_allowed(struct task_struct *p)
> +static inline void cpuset_print_current_mems_allowed(void)
> {
> }
>
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -2599,22 +2599,22 @@ int cpuset_mems_allowed_intersects(const struct task_struct *tsk1,
> }
>
> /**
> - * cpuset_print_task_mems_allowed - prints task's cpuset and mems_allowed
> - * @tsk: pointer to task_struct of some task.
> + * cpuset_print_current_mems_allowed - prints current's cpuset and mems_allowed
> *
> - * Description: Prints @task's name, cpuset name, and cached copy of its
> + * Description: Prints current's name, cpuset name, and cached copy of its
> * mems_allowed to the kernel log.
> */
> -void cpuset_print_task_mems_allowed(struct task_struct *tsk)
> +void cpuset_print_current_mems_allowed(void)
> {
> struct cgroup *cgrp;
>
> rcu_read_lock();
>
> - cgrp = task_cs(tsk)->css.cgroup;
> - pr_info("%s cpuset=", tsk->comm);
> + cgrp = task_cs(current)->css.cgroup;
> + pr_info("%s cpuset=", current->comm);
> pr_cont_cgroup_name(cgrp);
> - pr_cont(" mems_allowed=%*pbl\n", nodemask_pr_args(&tsk->mems_allowed));
> + pr_cont(" mems_allowed=%*pbl\n",
> + nodemask_pr_args(&current->mems_allowed));
>
> rcu_read_unlock();
> }
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -386,13 +386,11 @@ static void dump_tasks(struct mem_cgroup *memcg, const nodemask_t *nodemask)
> static void dump_header(struct oom_control *oc, struct task_struct *p,
> struct mem_cgroup *memcg)
> {
> - task_lock(current);
> pr_warning("%s invoked oom-killer: gfp_mask=0x%x, order=%d, "
> "oom_score_adj=%hd\n",
> current->comm, oc->gfp_mask, oc->order,
> current->signal->oom_score_adj);
> - cpuset_print_task_mems_allowed(current);
> - task_unlock(current);
> + cpuset_print_current_mems_allowed();
> dump_stack();
> if (memcg)
> mem_cgroup_print_oom_info(memcg, p);
> @@ -518,10 +516,8 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
> if (__ratelimit(&oom_rs))
> dump_header(oc, p, memcg);
>
> - task_lock(p);
> pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
> message, task_pid_nr(p), p->comm, points);
> - task_unlock(p);
>
> /*
> * If any of p's children has a different mm and is eligible for kill,
> @@ -586,10 +582,8 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
> if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
> continue;
>
> - task_lock(p); /* Protect ->comm from prctl() */
> pr_err("Kill process %d (%s) sharing same memory\n",
> task_pid_nr(p), p->comm);
> - task_unlock(p);
> do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
> }
> rcu_read_unlock();

--
Michal Hocko
SUSE Labs