Re: [PATCH 1/6] proc, oom: drop bogus task_lock and mm check

[Michal Hocko <mhocko@kernel.org>]

On Mon 30-05-16 19:43:24, Oleg Nesterov wrote:
> On 05/30, Michal Hocko wrote:
> >
> > both oom_adj_write and oom_score_adj_write are using task_lock,
> > check for task->mm and fail if it is NULL. This is not needed because
> > the oom_score_adj is per signal struct so we do not need mm at all.
> > The code has been introduced by 3d5992d2ac7d ("oom: add per-mm oom
> > disable count") but we do not do per-mm oom disable since c9f01245b6a7
> > ("oom: remove oom_disable_count").
> >
> > The task->mm check is even not correct because the current thread might
> > have exited but the thread group might be still alive - e.g. thread
> > group leader would lead that echo $VAL > /proc/pid/oom_score_adj would
> > always fail with EINVAL while /proc/pid/task/$other_tid/oom_score_adj
> > would succeed. This is unexpected at best.
> >
> > Remove the lock along with the check to fix the unexpected behavior
> > and also because there is not real need for the lock in the first place.
> 
> ACK

thanks!

> and we should also remove lock_task_sighand(). as for oom_adj_read() and
> oom_score_adj_read() we can just remove it right now; it was previously
> needed to ensure the task->signal != NULL, today this is always true.

OK, I will add the following patch to the series.
---
>From 952c464a31ffbe158233c4cc05f4b8a64384635c Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Tue, 31 May 2016 09:28:36 +0200
Subject: [PATCH] proc, oom: drop bogus sighand lock

Oleg has pointed out that can simplify both oom_adj_write and
oom_score_adj_write even further and drop the sighand lock. The only
purpose of the lock was to protect p->signal from going away but this
will not happen since ea6d290ca34c ("signals: make task_struct->signal
immutable/refcountable").

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 fs/proc/base.c | 20 ++------------------
 1 file changed, 2 insertions(+), 18 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index a6014e45c516..3761f107615a 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1057,7 +1057,6 @@ static ssize_t oom_adj_write(struct file *file, const char __user *buf,
 	struct task_struct *task;
 	char buffer[PROC_NUMBUF];
 	int oom_adj;
-	unsigned long flags;
 	int err;
 
 	memset(buffer, 0, sizeof(buffer));
@@ -1083,11 +1082,6 @@ static ssize_t oom_adj_write(struct file *file, const char __user *buf,
 		goto out;
 	}
 
-	if (!lock_task_sighand(task, &flags)) {
-		err = -ESRCH;
-		goto err_put_task;
-	}
-
 	/*
 	 * Scale /proc/pid/oom_score_adj appropriately ensuring that a maximum
 	 * value is always attainable.
@@ -1100,7 +1094,7 @@ static ssize_t oom_adj_write(struct file *file, const char __user *buf,
 	if (oom_adj < task->signal->oom_score_adj &&
 	    !capable(CAP_SYS_RESOURCE)) {
 		err = -EACCES;
-		goto err_sighand;
+		goto err_put_task;
 	}
 
 	/*
@@ -1113,8 +1107,6 @@ static ssize_t oom_adj_write(struct file *file, const char __user *buf,
 
 	task->signal->oom_score_adj = oom_adj;
 	trace_oom_score_adj_update(task);
-err_sighand:
-	unlock_task_sighand(task, &flags);
 err_put_task:
 	put_task_struct(task);
 out:
@@ -1152,7 +1144,6 @@ static ssize_t oom_score_adj_write(struct file *file, const char __user *buf,
 {
 	struct task_struct *task;
 	char buffer[PROC_NUMBUF];
-	unsigned long flags;
 	int oom_score_adj;
 	int err;
 
@@ -1179,15 +1170,10 @@ static ssize_t oom_score_adj_write(struct file *file, const char __user *buf,
 		goto out;
 	}
 
-	if (!lock_task_sighand(task, &flags)) {
-		err = -ESRCH;
-		goto err_put_task;
-	}
-
 	if ((short)oom_score_adj < task->signal->oom_score_adj_min &&
 			!capable(CAP_SYS_RESOURCE)) {
 		err = -EACCES;
-		goto err_sighand;
+		goto err_put_task;
 	}
 
 	task->signal->oom_score_adj = (short)oom_score_adj;
@@ -1195,8 +1181,6 @@ static ssize_t oom_score_adj_write(struct file *file, const char __user *buf,
 		task->signal->oom_score_adj_min = (short)oom_score_adj;
 	trace_oom_score_adj_update(task);
 
-err_sighand:
-	unlock_task_sighand(task, &flags);
 err_put_task:
 	put_task_struct(task);
 out:
-- 
2.8.1

-- 
Michal Hocko
SUSE Labs