Re: [PATCH v9] kernel/fork: beware of __put_task_struct calling context

[Oleg Nesterov <oleg@redhat.com>]

On 05/17, Wander Lairson Costa wrote:
>
> On Wed, May 17, 2023 at 12:26 PM Oleg Nesterov <oleg@redhat.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.