linux-kernel - Re: [PATCH v9] kernel/fork: beware of __put_task

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Message-ID: <20230529122256.GA588@redhat.com>
Date:   Mon, 29 May 2023 14:22:56 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Wander Lairson Costa <wander@...hat.com>
Cc:     "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        Brian Cain <bcain@...cinc.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Stafford Horne <shorne@...il.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrei Vagin <avagin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...nel.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Alexey Gladkov <legion@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Yang Shi <shy828301@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        Hu Chunyu <chuhu@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luis Goncalves <lgoncalv@...hat.com>
Subject: Re: [PATCH v9] kernel/fork: beware of __put_task_struct calling
 context

On 05/17, Wander Lairson Costa wrote:
>
> On Wed, May 17, 2023 at 12:26 PM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > LGTM but we still need to understand the possible problems with CONFIG_PROVE_RAW_LOCK_NESTING ...
> >
> > Again, I'll try to investigate when I have time although I am not sure I can really help.
> >
> > Perhaps you too can try to do this ? ;)
> >
>
> FWIW, I tested this patch with CONFIG_PROVE_LOCK_NESTING in RT and
> stock kernels. No splat happened.

Strange... FYI, I am running the kernel with this patch

	diff --git a/kernel/sys.c b/kernel/sys.c
	index 339fee3eff6a..3169cceddf3b 100644
	--- a/kernel/sys.c
	+++ b/kernel/sys.c
	@@ -2412,6 +2412,17 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
	 
		error = 0;
		switch (option) {
	+	case 666: {
	+		static DEFINE_SPINLOCK(l);
	+		static DEFINE_RAW_SPINLOCK(r);
	+
	+		raw_spin_lock(&r);
	+		spin_lock(&l);
	+		spin_unlock(&l);
	+		raw_spin_unlock(&r);
	+
	+		break;
	+	}
		case PR_SET_PDEATHSIG:
			if (!valid_signal(arg2)) {
				error = -EINVAL;

applied (because I am too lazy to compile a module ;) and

	# perl -e 'syscall 157,666'

triggers the lockdep bug

	=============================
	[ BUG: Invalid wait context ]
	6.4.0-rc2-00018-g4d6d4c7f541d-dirty #1176 Not tainted
	-----------------------------
	perl/35 is trying to lock:
	ffffffff81c4cc18 (l){....}-{3:3}, at: __do_sys_prctl+0x21b/0x87b
	other info that might help us debug this:
	context-{5:5}
	...

as expected.

Looks like your testing was wrong... Or maybe you missed another lockdep problem ?
Did you check dmesg? Perhaps lockdep detected another bug,say, even at boot time ?
In this case debug_locks_off() sets debug_locks = 0 and this disables lockdep.

Oleg.