proc, oom: drop bogus sighand lock

Oleg has pointed out that can simplify both oom_adj_{read,write} and
oom_score_adj_{read,write} even further and drop the sighand lock.  The
main 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").

The other role of the lock was to synchronize different writers,
especially those with CAP_SYS_RESOURCE.  Introduce a mutex for this
purpose.  Later patches will need this lock anyway.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Link: http://lkml.kernel.org/r/1466426628-15074-3-git-send-email-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 34 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 2a0f5ee9b623..f7dc1050387f 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1024,23 +1024,21 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
         char buffer[PROC_NUMBUF];
         int oom_adj = OOM_ADJUST_MIN;
         size_t len;
-        unsigned long flags;
 
         if (!task)
                 return -ESRCH;
-        if (lock_task_sighand(task, &flags)) {
-                if (task->signal->oom_score_adj == OOM_SCORE_ADJ_MAX)
-                        oom_adj = OOM_ADJUST_MAX;
-                else
-                        oom_adj = (task->signal->oom_score_adj * -OOM_DISABLE) /
-                                  OOM_SCORE_ADJ_MAX;
-                unlock_task_sighand(task, &flags);
-        }
+        if (task->signal->oom_score_adj == OOM_SCORE_ADJ_MAX)
+                oom_adj = OOM_ADJUST_MAX;
+        else
+                oom_adj = (task->signal->oom_score_adj * -OOM_DISABLE) /
+                          OOM_SCORE_ADJ_MAX;
         put_task_struct(task);
         len = snprintf(buffer, sizeof(buffer), "%d\n", oom_adj);
         return simple_read_from_buffer(buf, count, ppos, buffer, len);
 }
 
+static DEFINE_MUTEX(oom_adj_mutex);
+
 /*
  * /proc/pid/oom_adj exists solely for backwards compatibility with previous
  * kernels.  The effective policy is defined by oom_score_adj, which has a
@@ -1057,7 +1055,6 @@ 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 +1080,6 @@ 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.
@@ -1097,10 +1089,11 @@ static ssize_t oom_adj_write(struct file *file, const char __user *buf,
         else
                 oom_adj = (oom_adj * OOM_SCORE_ADJ_MAX) / -OOM_DISABLE;
 
+        mutex_lock(&oom_adj_mutex);
         if (oom_adj < task->signal->oom_score_adj &&
             !capable(CAP_SYS_RESOURCE)) {
                 err = -EACCES;
-                goto err_sighand;
+                goto err_unlock;
         }
 
         /*
@@ -1113,9 +1106,8 @@ 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:
+err_unlock:
+        mutex_unlock(&oom_adj_mutex);
         put_task_struct(task);
 out:
         return err < 0 ? err : count;
@@ -1133,15 +1125,11 @@ static ssize_t oom_score_adj_read(struct file *file, char __user *buf,
         struct task_struct *task = get_proc_task(file_inode(file));
         char buffer[PROC_NUMBUF];
         short oom_score_adj = OOM_SCORE_ADJ_MIN;
-        unsigned long flags;
         size_t len;
 
         if (!task)
                 return -ESRCH;
-        if (lock_task_sighand(task, &flags)) {
-                oom_score_adj = task->signal->oom_score_adj;
-                unlock_task_sighand(task, &flags);
-        }
+        oom_score_adj = task->signal->oom_score_adj;
         put_task_struct(task);
         len = snprintf(buffer, sizeof(buffer), "%hd\n", oom_score_adj);
         return simple_read_from_buffer(buf, count, ppos, buffer, len);
@@ -1152,7 +1140,6 @@ 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;
 
@@ -1179,25 +1166,21 @@ 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;
-        }
-
+        mutex_lock(&oom_adj_mutex);
         if ((short)oom_score_adj < task->signal->oom_score_adj_min &&
                         !capable(CAP_SYS_RESOURCE)) {
                 err = -EACCES;
-                goto err_sighand;
+                goto err_unlock;
         }
 
         task->signal->oom_score_adj = (short)oom_score_adj;
         if (has_capability_noaudit(current, CAP_SYS_RESOURCE))
                 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:
+err_unlock:
+        mutex_unlock(&oom_adj_mutex);
         put_task_struct(task);
 out:
         return err < 0 ? err : count;