mm, oom: remove task_lock protecting comm printing

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.

Change-Id: I89f64666a1db5d414aa53862fd6b665bbb8125bc
Signed-off-by: David Rientjes <rientjes@google.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 11 insertions, 19 deletions

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 7bc96dce8..660ccb535 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -86,7 +86,7 @@ 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);
 
 /*
  * get_mems_allowed is required when making decisions involving mems_allowed
@@ -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
index ab0be4565..f3a417c1d 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -2601,28 +2601,26 @@ int cpuset_mems_allowed_intersects(const struct task_struct *tsk1,
 #define CPUSET_NODELIST_LEN	(256)
 
 /**
- * cpuset_print_task_mems_allowed - prints task's cpuset and mems_allowed
- * @task: 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
- * mems_allowed to the kernel log.  Must hold task_lock(task) to allow
- * dereferencing task_cs(task).
+ * 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)
 {
 	 /* Statically allocated to prevent using excess stack. */
 	static char cpuset_nodelist[CPUSET_NODELIST_LEN];
 	static DEFINE_SPINLOCK(cpuset_buffer_lock);
 
-	struct cgroup *cgrp = task_cs(tsk)->css.cgroup;
+	struct cgroup *cgrp = task_cs(current)->css.cgroup;
 
 	rcu_read_lock();
 	spin_lock(&cpuset_buffer_lock);
 
 	nodelist_scnprintf(cpuset_nodelist, CPUSET_NODELIST_LEN,
-			   tsk->mems_allowed);
-	printk(KERN_INFO "%s cpuset=%s mems_allowed=%s\n",
-	       tsk->comm, cgroup_name(cgrp), cpuset_nodelist);
+			   current->mems_allowed);
+	pr_info("%s cpuset=%s mems_allowed=%s\n",
+	       current->comm, cgroup_name(cgrp), cpuset_nodelist);
 
 	spin_unlock(&cpuset_buffer_lock);
 	rcu_read_unlock();
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 4b7e9037d..5933c3c15 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -399,13 +399,11 @@ void dump_tasks(const struct mem_cgroup *memcg, const nodemask_t *nodemask)
 static void dump_header(struct task_struct *p, gfp_t gfp_mask, int order,
 			struct mem_cgroup *memcg, const nodemask_t *nodemask)
 {
-	task_lock(current);
 	pr_warning("%s invoked oom-killer: gfp_mask=0x%x, order=%d, "
 		"oom_score_adj=%hd\n",
 		current->comm, gfp_mask, 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);
@@ -533,10 +531,8 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	if (__ratelimit(&oom_rs))
 		dump_header(p, gfp_mask, order, memcg, nodemask);
 
-	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,
@@ -629,10 +625,8 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 			if (fatal_signal_pending(p))
 				continue;
 
-			task_lock(p);	/* Protect ->comm from prctl() */
 			pr_info("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();