Re: [PATCH v2] kernel/signal: Signal-based pre-coredump notification

[Enke Chen]

Hi, Olge:

On 10/24/18 6:52 AM, Oleg Nesterov wrote:
> On 10/23, Enke Chen wrote:
>>
>>>> +	/*
>>>> +	 * Send the pre-coredump signal to the parent if requested.
>>>> +	 */
>>>> +	read_lock(&tasklist_lock);
>>>> +	notify = do_notify_parent_predump(current);
>>>> +	read_unlock(&tasklist_lock);
>>>> +	if (notify)
>>>> +		cond_resched();
>>>
>>> Hmm. I do not understand why do we need cond_resched(). And even if we need it,
>>> why we can't call it unconditionally?
>>
>> Remember the goal is to allow the parent (e.g., a process manager) to take early
>> action. The "yield" before doing coredump will help.
> 
> I don't see how can it actually help...
> 
> cond_resched() is nop if CONFIG_PREEMPT or should_resched() == 0.
> 
> and the coredumping thread will certainly need to sleep/wait anyway.

I am really surprised by this - cond_resched() is used in many places and it actually
does not do anything w/o CONFIG_PREEMPT.

Will remove.

> 
>>> And once again, SIGCHLD/SIGUSR do not queue, this means that PR_SET_PREDUMP_SIG
>>> is pointless if you have 2 or more children.
>>
>> Hmm, could you point me to the code where SIGCHLD/SIGUSR is treated differently
>> w.r.t. queuing?  That does not sound right to me.
> 
> see the legacy_queue() check. Any signal < SIGRTMIN do not queue. IOW, if SIGCHLD
> is already pending, then next SIGCHLD is simply ignored.

Got it. This means that a distinct signal (in particular a RT signal) would be more
preferred. This is what it is done in our application. You earlier suggestion about
removing the signal limitation makes a lot sense to me now.

Given that a distinct signal is more preferred, I am wondering if I should just remove
CLD_PREDUMP from the patch.

Thanks.  -- Enke