linux-kernel - Re: Linux 3.7-rc3

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Message-ID: <CA+55aFx+w2hFifGTr_u76D9d_sDKxKJBDd2L4p0GVk-RXCczMA@mail.gmail.com>
Date:	Fri, 2 Nov 2012 16:10:34 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Daniel Vetter <daniel.vetter@...ll.ch>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	David Airlie <airlied@...ux.ie>, Michal Hocko <mhocko@...e.cz>,
	Jiri Kosina <jkosina@...e.cz>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Linux 3.7-rc3

On Fri, Nov 2, 2012 at 3:43 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>
> Well, not everything is rosy in the suspend land, though.  This is a
> failure to freeze khubd during the second in a row attempt to suspend to
> RAM (your current tree):

Ugh. So khubd is blocked in usb_start_wait_urb(), and apparently the
timeout for that block is longer than the freezing timeout.

There's a comment about why khubd needs to be freezable, but I wonder
if that whole thing isn't doing something wrong. Causing the suspend
to fail is definitely always the wrong thing.

Greg?

> [  125.780766] [ INFO: suspicious RCU usage. ]
> [  125.780804] 3.7.0-rc3+ #988 Not tainted
> [  125.780838] -------------------------------
> [  125.780875] /home/rafael/src/linux/kernel/sched/core.c:4497 suspicious rcu_dereference_check() usage!

Heh. The RCU usage is from the debug printout from sched_show_task(),
so it's "related", but it's a totally independent issue.

It's apparently because we've not done a "rcu_read_lock()" around that
sequence, but I seriously doubt we care. But it's technically a real
bug - even if the fix might be to just not print out the parent pid
(or to just ignore the bug and turn the rcu dereference into an
ACCESS_ONCE() or something.

Ingo, Peter, any comments about that sched/core.c:4497 RCU usage?

                Linus
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