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

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Date:   Wed, 24 Oct 2018 14:56:12 -0700
From:   Enke Chen <enkechen@...co.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Helge Deller <deller@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian@...uner.io>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Dave Martin <Dave.Martin@....com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Roman Gushchin <guro@...com>,
        Marcos Paulo de Souza <marcos.souza.org@...il.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        "Victor Kamensky (kamensky)" <kamensky@...co.com>,
        xe-linux-external@...co.com, Stefan Strogin <sstrogin@...co.com>,
        Enke Chen <enkechen@...co.com>
Subject: Re: [PATCH v2] kernel/signal: Signal-based pre-coredump notification

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