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Date: Mon, 9 Nov 2015 19:55:06 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Roland McGrath <roland@hack.frob.com>, Tejun Heo <tj@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: ptrace() hangs on attempt to seize/attach stopped & frozen task
Message-ID: <20151109185506.GA22744@redhat.com>
References: <5640B7F2.70406@virtuozzo.com>
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Hi,

On 11/09, Andrey Ryabinin wrote:
>
> Hi,
>
> So, the ptrace() hangs if we try to attach to stopped task from freezing cgroup.
> It seems this was introduced by 5d8f72b55c2756("freezer: change ptrace_stop/do_signal_stop to use freezable_schedule()").

quite possible...

> See below for the exact scenario and small script to reproduce this.
>
>
> Tracee:                                                                 Tracer:
> static bool do_signal_stop(int signr)
> 	__set_current_state(TASK_STOPPED);
> 	freezable_schedule();
> 		freezer_do_not_count();
> 		schedule(); /* waiting for wake up */
>
> 									ptrace_attach()
> 										if (task_is_stopped(task) &&
> 										    task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING))
> 											signal_wake_up_state(task, __TASK_STOPPED);
>
> 		/* woken up by ptrace_attach() */
> 		freezer_count();
> 			__refrigerator()
> 										/* And here we will hang, because tracee is now frozen in __refrigerator() */
> 										wait_on_bit(&task->jobctl, JOBCTL_TRAPPING_BIT,
> 												TASK_UNINTERRUPTIBLE);

Thanks. All I can say I never liked this wait_on_bit() ;)

I need to think, but *at first glance* we can move this wait-for-stopped-
traced-transition into do_wait() path, and this way clear_jobctl_trapping()
can use __wake_up_parent(). Perhaps we just need to modify task_stopped_code()
to take JOBCTL_TRAPPING into account...

Sure, debugger will block in sys_wait() after PTRACE_ATTACH/SEIZE. But this
does not really differ from the case when the tracee was already frozen;
SIGSTOP sent by ATTACH or PTRACE_INTERRUPT, so debugger will equally block
in do_wait() until the tracee is unfrozen.

Oleg.