linux-kernel - Re: [PATCH 2/2] lockdep: check that no locks held at freeze time

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Date:	Sun, 5 May 2013 00:57:15 +0200
From:	Pavel Machek <pavel@....cz>
To:	Colin Cross <ccross@...roid.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Len Brown <len.brown@...el.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mandeep Singh Baines <msb@...omium.org>,
	Paul Walmsley <paul@...an.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-nfs@...r.kernel.org,
	Linux PM list <linux-pm@...r.kernel.org>,
	netdev@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Ben Chan <benchan@...omium.org>
Subject: Re: [PATCH 2/2] lockdep: check that no locks held at freeze time

On Sat 2013-05-04 13:27:23, Colin Cross wrote:
> On Sat, May 4, 2013 at 6:04 AM, Pavel Machek <pavel@....cz> wrote:
> > On Fri 2013-05-03 14:04:10, Colin Cross wrote:
> >> From: Mandeep Singh Baines <msb@...omium.org>
> >>
> >> We shouldn't try_to_freeze if locks are held.  Holding a lock can cause a
> >> deadlock if the lock is later acquired in the suspend or hibernate path
> >> (e.g.  by dpm).  Holding a lock can also cause a deadlock in the case of
> >> cgroup_freezer if a lock is held inside a frozen cgroup that is later
> >> acquired by a process outside that group.
> >
> > Ok, but this does not explain why
> >
> >> --- a/include/linux/debug_locks.h
> >> +++ b/include/linux/debug_locks.h
> >> @@ -51,7 +51,7 @@ struct task_struct;
> >>  extern void debug_show_all_locks(void);
> >>  extern void debug_show_held_locks(struct task_struct *task);
> >>  extern void debug_check_no_locks_freed(const void *from, unsigned long len);
> >> -extern void debug_check_no_locks_held(struct task_struct *task);
> >> +extern void debug_check_no_locks_held(void);
> >>  #else
> >>  static inline void debug_show_all_locks(void)
> >>  {
> >
> > Removing task_struct argument from  those functions is good idea?
> 
> This is an existing patch that was merged in 3.9 and then reverted
> again, so it has already been reviewed and accepted once.

Well, it was also reverted once :-).

> >> --- a/kernel/exit.c
> >> +++ b/kernel/exit.c
> >> @@ -835,7 +835,7 @@ void do_exit(long code)
> >>       /*
> >>        * Make sure we are holding no locks:
> >>        */
> >> -     debug_check_no_locks_held(tsk);
> >> +     debug_check_no_locks_held();
> >
> > Is task guaranteed == current?
> 
> Yes, the first line of do_exit is:
>         struct task_struct *tsk = current;

Aha, I understand it now.

Accessing current is slower than local variable. So your "new" code
will work but will be slower. Please revert this part.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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