signal: Turn SIGNAL_STOP_DEQUEUED into GROUP_STOP_DEQUEUED

This patch moves SIGNAL_STOP_DEQUEUED from signal_struct->flags to
task_struct->group_stop, and thus makes it per-thread.

Like SIGNAL_STOP_DEQUEUED, GROUP_STOP_DEQUEUED can be false-positive
after return from get_signal_to_deliver(), this is fine. The only
purpose of this bit is: we can drop ->siglock after __dequeue_signal()
returns the sig_kernel_stop() signal and before we call
do_signal_stop(), in this case we must not miss SIGCONT if it comes in
between.

But, unlike SIGNAL_STOP_DEQUEUED, GROUP_STOP_DEQUEUED can not be
false-positive in do_signal_stop() if multiple threads dequeue the
sig_kernel_stop() signal at the same time.

Consider two threads T1 and T2, SIGTTIN has a hanlder.

	- T1 dequeues SIGTSTP and sets SIGNAL_STOP_DEQUEUED, then
	  it drops ->siglock

	- SIGCONT comes and clears SIGNAL_STOP_DEQUEUED, SIGTSTP
	  should be cancelled.

	- T2 dequeues SIGTTIN and sets SIGNAL_STOP_DEQUEUED again.
	  Since we have a handler we should not stop, T2 returns
	  to usermode to run the handler.

	- T1 continues, calls do_signal_stop() and wrongly starts
	  the group stop because SIGNAL_STOP_DEQUEUED was restored
	  in between.

With or without this change:

	- we need to do something with ptrace_signal() which can
	  return SIGSTOP, but this needs another discussion

	- SIGSTOP can be lost if it races with the mt exec, will
	  be fixed later.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 4 insertions, 10 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index e9abc69dc0d8..4f7312b49b2d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -254,7 +254,8 @@ static void task_clear_group_stop_trapping(struct task_struct *task)
  */
 void task_clear_group_stop_pending(struct task_struct *task)
 {
-        task->group_stop &= ~(GROUP_STOP_PENDING | GROUP_STOP_CONSUME);
+        task->group_stop &= ~(GROUP_STOP_PENDING | GROUP_STOP_CONSUME |
+                              GROUP_STOP_DEQUEUED);
 }
 
 /**
@@ -602,7 +603,7 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
                  * is to alert stop-signal processing code when another
                  * processor has come along and cleared the flag.
                  */
-                tsk->signal->flags |= SIGNAL_STOP_DEQUEUED;
+                current->group_stop |= GROUP_STOP_DEQUEUED;
         }
         if ((info->si_code & __SI_MASK) == __SI_TIMER && info->si_sys_private) {
                 /*
@@ -821,13 +822,6 @@ static int prepare_signal(int sig, struct task_struct *p, int from_ancestor_ns)
                         signal->flags = why | SIGNAL_STOP_CONTINUED;
                         signal->group_stop_count = 0;
                         signal->group_exit_code = 0;
-                } else {
-                        /*
-                         * We are not stopped, but there could be a stop
-                         * signal in the middle of being processed after
-                         * being removed from the queue.  Clear that too.
-                         */
-                        signal->flags &= ~SIGNAL_STOP_DEQUEUED;
                 }
         }
 
@@ -1855,7 +1849,7 @@ static int do_signal_stop(int signr)
                 /* signr will be recorded in task->group_stop for retries */
                 WARN_ON_ONCE(signr & ~GROUP_STOP_SIGMASK);
 
-                if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED) ||
+                if (!likely(current->group_stop & GROUP_STOP_DEQUEUED) ||
                     unlikely(signal_group_exit(sig)))
                         return 0;
                 /*