linux-kernel - Re: [PATCH v4] lockdep: check that no locks held at freeze time

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Date:	Wed, 20 Feb 2013 16:20:15 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mandeep Singh Baines <msb@...omium.org>
Cc:	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Tejun Heo <tj@...nel.org>, "Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [PATCH v4] lockdep: check that no locks held at freeze time

On Wed, 20 Feb 2013 16:17:39 -0800
Mandeep Singh Baines <msb@...omium.org> wrote:

> On Wed, Feb 20, 2013 at 3:24 PM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
> > On Wed, 20 Feb 2013 15:17:16 -0800
> > Mandeep Singh Baines <msb@...omium.org> wrote:
> >
> >> We shouldn't try_to_freeze if locks are held.
> >>
> >> ...
> >>
> >> @@ -43,6 +44,9 @@ extern void thaw_kernel_threads(void);
> >>
> >> +     if (!(current->flags & PF_NOFREEZE))
> >> +             debug_check_no_locks_held(current,
> >> +
> >>                                         "lock held while trying to freeze");
> >> ...
> >>
> >> +     debug_check_no_locks_held(tsk, "lock held at task exit time");
> >
> > There doesn't seem much point in adding the `msg' to
> > debug_check_no_locks_held() - the dump_stack() in
> > print_held_locks_bug() will tell us the same thing.  Maybe just change
> 
> dump_stack() can be confusing when there is inlining. On occasion I've
> looked at the wrong mutex_lock, for example, when there was another
> mutex_lock that was inlined. Of course, you can start objdump and
> verify the offsets. But that requires that you have the object file.
> You could have a try_to_freeze added to do_exit. I was thinking of
> adding another locks_held in the return from syscall path.

Backtraces aren't *that* bad.  We'll easily be able to tell which of
the two callsites triggered the trace.

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