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Commit ee77f075 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Tejun Heo
Browse files

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: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 780006ea

include/linux/sched.h

+3 −3
Original line number Diff line number Diff line
@@ -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);

kernel/signal.c

+4 −10
Original line number Diff line number Diff line
@@ -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;

		/*