linux-kernel - Re: [PATCH v2 03/14] notifiers: Assert that RCU is watching in notify

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Message-ID: <CALCETrW9owchn_kf9aC5HQ76a61H4S=jdY60Hpg3D43Nn+m7LQ@mail.gmail.com>
Date:	Mon, 22 Jun 2015 10:37:39 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Kees Cook <keescook@...omium.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Brian Gerst <brgerst@...il.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	X86 ML <x86@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v2 03/14] notifiers: Assert that RCU is watching in notify_die

On Mon, Jun 22, 2015 at 10:24 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Mon, Jun 22, 2015 at 10:03:30AM -0700, Andy Lutomirski wrote:
>> The rcu_lockdep_assert should be merely a warning, not a full OOPS.
>
> It is still pretty huge, see below.
>
>> I think that, if rcu_lockdep_assert hangs, then we should fix that
>> rather than avoiding debugging checks.
>
> The RCU assertion firing might be unrelated to the oops happening and
> could prevent us from seeing the real splat.
>
> [    0.048815]
> [    0.050493] ===============================
> [    0.052005] [ INFO: suspicious RCU usage. ]
> [    0.056007] 4.1.0-rc8+ #4 Not tainted
> [    0.060005] -------------------------------
> [    0.064005] arch/x86/kernel/cpu/amd.c:677 BOINK!
> [    0.066758]
> [    0.066758] other info that might help us debug this:
> [    0.066758]
> [    0.068006]
> [    0.068006] rcu_scheduler_active = 0, debug_locks = 0
> [    0.072005] no locks held by swapper/0/0.
> [    0.076005]
> [    0.076005] stack backtrace:
> [    0.080006] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.1.0-rc8+ #4
> [    0.083331] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
> [    0.084021]  0000000000000000 ffffffff81967eb8 ffffffff816709c7 0000000000000000
> [    0.092005]  ffffffff81975580 ffffffff81967ee8 ffffffff8109e8cd 0000000000000000
> [    0.097227]  ffffffff81a3aec0 ffffffff81cad9c0 ffffffff81cb42c0 ffffffff81967f38
> [    0.104005] Call Trace:
> [    0.106021]  [<ffffffff816709c7>] dump_stack+0x4f/0x7b
> [    0.108007]  [<ffffffff8109e8cd>] lockdep_rcu_suspicious+0xfd/0x130
> [    0.112007]  [<ffffffff81017f74>] init_amd+0x34/0x560
> [    0.116007]  [<ffffffff810164e2>] identify_cpu+0x242/0x3b0
> [    0.119068]  [<ffffffff81c27172>] identify_boot_cpu+0x10/0x7e
> [    0.120006]  [<ffffffff81c27214>] check_bugs+0x9/0x2d
> [    0.124007]  [<ffffffff81c1fe8e>] start_kernel+0x40e/0x425
> [    0.128007]  [<ffffffff81c1f495>] x86_64_start_reservations+0x2a/0x2c
> [    0.132009]  [<ffffffff81c1f582>] x86_64_start_kernel+0xeb/0xef

But if we OOPS, we'll OOPS after the lockdep splat and the lockdep
splat will scroll off the screen, right?  Am I missing something here?

notify_die is called before the actual OOPS code is invoked in traps.c.

--Andy
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