Commit 90bc8d8b authored Apr 02, 2009 by Oleg Nesterov Committed by Linus Torvalds Apr 02, 2009

do_wait: fix waiting for the group stop with the dead leader

do_wait(WSTOPPED) assumes that p->state must be == TASK_STOPPED, this is
not true if the leader is already dead.  Check SIGNAL_STOP_STOPPED instead
and use signal->group_exit_code.

Trivial test-case:

	void *tfunc(void *arg)
	{
		pause();
		return NULL;
	}

	int main(void)
	{
		pthread_t thr;
		pthread_create(&thr, NULL, tfunc, NULL);
		pthread_exit(NULL);
		return 0;
	}

It doesn't react to ^Z (and then to ^C or ^\). The task is stopped, but
bash can't see this.

The bug is very old, and it was reported multiple times. This patch was sent
more than a year ago (http://marc.info/?t=119713920000003

) but it was ignored.

This change also fixes other oddities (but not all) in this area.  For
example, before this patch:

	$ sleep 100
	^Z
	[1]+  Stopped                 sleep 100
	$ strace -p `pidof sleep`
	Process 11442 attached - interrupt to quit

strace hangs in do_wait(), because ->exit_code was already consumed by
bash.  After this patch, strace happily proceeds:

	--- SIGTSTP (Stopped) @ 0 (0) ---
	restart_syscall(<... resuming interrupted call ...>

To me, this looks much more "natural" and correct.

Another example.  Let's suppose we have the main thread M and sub-thread
T, the process is stopped, and its parent did wait(WSTOPPED).  Now we can
ptrace T but not M.  This looks at least strange to me.

Imho, do_wait() should not confuse the per-thread ptrace stops with the
per-process job control stops.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Kaz Kylheku <kkylheku@gmail.com>
Cc: Michael Kerrisk <mtk.manpages@googlemail.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 6d7b2f5f

kernel/exit.c

+18 −12

Original line number	Diff line number	Diff line
		@@ -1417,6 +1417,18 @@ static int wait_task_zombie(struct task_struct *p, int options,
		return retval;
		}

		static int task_stopped_code(struct task_struct p, bool ptrace)
		{
		if (ptrace) {
		if (task_is_stopped_or_traced(p))
		return &p->exit_code;
		} else {
		if (p->signal->flags & SIGNAL_STOP_STOPPED)
		return &p->signal->group_exit_code;
		}
		return NULL;
		}

		/*
		* Handle sys_wait4 work for one task in state TASK_STOPPED. We hold
		* read_lock(&tasklist_lock) on entry. If we return zero, we still hold
		@@ -1427,7 +1439,7 @@ static int wait_task_stopped(int ptrace, struct task_struct *p,
		int options, struct siginfo __user *infop,
		int __user stat_addr, struct rusage __user ru)
		{
		int retval, exit_code, why;
		int retval, exit_code, *p_code, why;
		uid_t uid = 0; /* unneeded, required by compiler */
		pid_t pid;

		@@ -1437,22 +1449,16 @@ static int wait_task_stopped(int ptrace, struct task_struct *p,
		exit_code = 0;
		spin_lock_irq(&p->sighand->siglock);

		if (unlikely(!task_is_stopped_or_traced(p)))
		p_code = task_stopped_code(p, ptrace);
		if (unlikely(!p_code))
		goto unlock_sig;

		if (!ptrace && p->signal->group_stop_count > 0)
		/*
		* A group stop is in progress and this is the group leader.
		* We won't report until all threads have stopped.
		*/
		goto unlock_sig;

		exit_code = p->exit_code;
		exit_code = *p_code;
		if (!exit_code)
		goto unlock_sig;

		if (!unlikely(options & WNOWAIT))
		p->exit_code = 0;
		*p_code = 0;

		/* don't need the RCU readlock here as we're holding a spinlock */
		uid = __task_cred(p)->uid;
		@@ -1608,7 +1614,7 @@ static int wait_consider_task(struct task_struct *parent, int ptrace,
		*/
		*notask_error = 0;

		if (task_is_stopped_or_traced(p))
		if (task_stopped_code(p, ptrace))
		return wait_task_stopped(ptrace, p, options,
		infop, stat_addr, ru);

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