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

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Date:   Wed, 24 Oct 2018 15:52:13 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Enke Chen <enkechen@...co.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>
Subject: Re: [PATCH v2] kernel/signal: Signal-based pre-coredump notification

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.

> > 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.

Oleg.