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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>
This commit is contained in:
Oleg Nesterov 2011-04-01 20:12:38 +02:00 committed by Tejun Heo
parent 780006eac2
commit ee77f07592
2 changed files with 7 additions and 13 deletions
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6
include/linux/sched.h
View File

@ -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);

14
kernel/signal.c
View File

@ -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;
/*