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

David Rientjes
2015-09-22T23:30:16Z

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.

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