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, 3 insertions, 3 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 456d80e..8cef82d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -652,9 +652,8 @@ struct signal_struct {
  * Bits in flags field of signal_struct.
  */
 #define SIGNAL_STOP_STOPPED	0x00000001 /* job control stop in effect */
-#define SIGNAL_STOP_DEQUEUED	0x00000002 /* stop signal dequeued */
-#define SIGNAL_STOP_CONTINUED	0x00000004 /* SIGCONT since WCONTINUED reap */
-#define SIGNAL_GROUP_EXIT	0x00000008 /* group exit in progress */
+#define SIGNAL_STOP_CONTINUED	0x00000002 /* SIGCONT since WCONTINUED reap */
+#define SIGNAL_GROUP_EXIT	0x00000004 /* group exit in progress */
 /*
  * Pending notifications to parent.
  */
@@ -1779,6 +1778,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
 #define GROUP_STOP_PENDING	(1 << 16) /* task should stop for group stop */
 #define GROUP_STOP_CONSUME	(1 << 17) /* consume group stop count */
 #define GROUP_STOP_TRAPPING	(1 << 18) /* switching from STOPPED to TRACED */
+#define GROUP_STOP_DEQUEUED	(1 << 19) /* stop signal dequeued */
 
 extern void task_clear_group_stop_pending(struct task_struct *task);