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Date:   Tue, 16 Oct 2018 16:14:06 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Enke Chen <enkechen@cisco.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        Khalid Aziz <khalid.aziz@oracle.com>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        Helge Deller <deller@gmx.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Catalin Marinas <catalin.marinas@arm.com>,
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        "Victor Kamensky (kamensky)" <kamensky@cisco.com>,
        xe-linux-external@cisco.com, Stefan Strogin <sstrogin@cisco.com>,
        Eugene Syromiatnikov <esyr@redhat.com>
Subject: Re: [PATCH] kernel/signal: Signal-based pre-coredump notification
Message-ID: <20181016141405.GA22045@redhat.com>
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 <20181015120521.GA10146@redhat.com>
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On 10/15, Enke Chen wrote:
>
> > I don't understand why we need valid_predump_signal() at all.
>
> Most of the signals have well-defined semantics, and would not be appropriate
> for this purpose.

you are going to change the rules anyway.

> That is why it is limited to only SIGCHLD, SIGUSR1, SIGUSR2.

Which do not queue. So the parent won't get the 2nd signal if 2 children
crash at the same time.

> >>  		if (sig_kernel_coredump(signr)) {
> >> +			/*
> >> +			 * Notify the parent prior to the coredump if the
> >> +			 * parent is interested in such a notificaiton.
> >> +			 */
> >> +			int p_sig = current->real_parent->predump_signal;
> >> +
> >> +			if (valid_predump_signal(p_sig)) {
> >> +				read_lock(&tasklist_lock);
> >> +				do_notify_parent_predump(current);
> >> +				read_unlock(&tasklist_lock);
> >> +				cond_resched();
> >
> > perhaps this should be called by do_coredump() after coredump_wait() kills
> > all the sub-threads?
>
> proc_coredump_connector(current) is located here, they should stay together.

Why?

Once again, other threads are still alive. So if the parent restarts the service
after it recieves -predump_signal, the new process can "race" with the old thread.

Oleg.