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

[Pavel Machek]

On Sat 2013-05-04 13:27:23, Colin Cross wrote:
> On Sat, May 4, 2013 at 6:04 AM, Pavel Machek <pavel@xxxxxx> wrote:
> > On Fri 2013-05-03 14:04:10, Colin Cross wrote:
> >> From: Mandeep Singh Baines <msb@xxxxxxxxxxxx>
> >>
> >> 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
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