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Commit d12619b5 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

fix group stop with exit race 



do_signal_stop() counts all sub-thread and sets ->group_stop_count
accordingly.  Every thread should decrement ->group_stop_count and stop,
the last one should notify the parent.

However a sub-thread can exit before it notices the signal_pending(), or it
may be somewhere in do_exit() already.  In that case the group stop never
finishes properly.

Note: this is a minimal fix, we can add some optimizations later.  Say we
can return quickly if thread_group_empty().  Also, we can move some signal
related code from exit_notify() to exit_signals().

Signed-off-by: default avatarOleg Nesterov <oleg@tv-sign.ru>
Acked-by: default avatarDavide Libenzi <davidel@xmailserver.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 430c6231

include/linux/signal.h

+1 −0
Original line number Diff line number Diff line
@@ -241,6 +241,7 @@ extern int show_unhandled_signals; 


struct pt_regs;

extern int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka, struct pt_regs *regs, void *cookie);

extern void exit_signals(struct task_struct *tsk);



extern struct kmem_cache *sighand_cachep;

kernel/exit.c

+1 −1
Original line number Diff line number Diff line
@@ -947,7 +947,7 @@ fastcall NORET_TYPE void do_exit(long code) 
		schedule();

	}



	tsk->flags |= PF_EXITING;

	exit_signals(tsk);  /* sets PF_EXITING */

	/*

	 * tsk->flags are checked in the futex code to protect against

	 * an exiting task cleaning up the robust pi futexes.

kernel/signal.c

+26 −1
Original line number Diff line number Diff line
@@ -1739,7 +1739,7 @@ static int do_signal_stop(int signr) 
			 * stop is always done with the siglock held,

			 * so this check has no races.

			 */

			if (!t->exit_state &&

			if (!(t->flags & PF_EXITING) &&

			    !task_is_stopped_or_traced(t)) {

				stop_count++;

				signal_wake_up(t, 0);
@@ -1900,6 +1900,31 @@ relock: 
	return signr;

}



void exit_signals(struct task_struct *tsk)

{

	int group_stop = 0;



	spin_lock_irq(&tsk->sighand->siglock);

	if (unlikely(tsk->signal->group_stop_count) &&

			!--tsk->signal->group_stop_count) {

		tsk->signal->flags = SIGNAL_STOP_STOPPED;

		group_stop = 1;

	}



	/*

	 * From now this task is not visible for group-wide signals,

	 * see wants_signal(), do_signal_stop().

	 */

	tsk->flags |= PF_EXITING;

	spin_unlock_irq(&tsk->sighand->siglock);



	if (unlikely(group_stop)) {

		read_lock(&tasklist_lock);

		do_notify_parent_cldstop(tsk, CLD_STOPPED);

		read_unlock(&tasklist_lock);

	}

}



EXPORT_SYMBOL(recalc_sigpending);

EXPORT_SYMBOL_GPL(dequeue_signal);

EXPORT_SYMBOL(flush_signals);