signals: send_sig_info: don't take tasklist_lock

The comment in send_sig_info() is wrong, tasklist_lock can't help.

The caller must ensure the task can't go away, otherwise ->sighand can be NULL
even before we take the lock.

p->sighand could be changed by exec(), but I can't imagine how it is possible
to prevent exit(), but not exec().

Since the things seem to work, I assume all callers are correct.  However,
drm_vbl_send_signals() looks broken.  block_all_signals() which is solely used
by drm is definitely broken.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 10 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 13371d17358d..17859f0d8411 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1138,8 +1138,7 @@ static int kill_something_info(int sig, struct siginfo *info, int pid)
  */
 
 /*
- * These two are the most common entry points.  They send a signal
- * just to the specific thread.
+ * The caller must ensure the task can't exit.
  */
 int
 send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
@@ -1154,17 +1153,9 @@ send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
         if (!valid_signal(sig))
                 return -EINVAL;
 
-        /*
-         * We need the tasklist lock even for the specific
-         * thread case (when we don't need to follow the group
-         * lists) in order to avoid races with "p->sighand"
-         * going away or changing from under us.
-         */
-        read_lock(&tasklist_lock);  
         spin_lock_irqsave(&p->sighand->siglock, flags);
         ret = specific_send_sig_info(sig, info, p);
         spin_unlock_irqrestore(&p->sighand->siglock, flags);
-        read_unlock(&tasklist_lock);
         return ret;
 }