linux-kernel - Re: [PATCH 2/2] mutex: use spin_[un]lock instead of arch_spin

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Message-ID: <20130124101450.GA27602@gmail.com>
Date:	Thu, 24 Jan 2013 11:14:50 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/2] mutex: use spin_[un]lock instead of
 arch_spin_[un]lock


* Yuanhan Liu <yuanhan.liu@...ux.intel.com> wrote:

> On Thu, Jan 24, 2013 at 10:58:07AM +0100, Ingo Molnar wrote:
> > 
> > * Yuanhan Liu <yuanhan.liu@...ux.intel.com> wrote:
> > 
> > > Use spin_[un]lock instead of arch_spin_[un]lock in mutex-debug.h so
> > > that we can collect the lock statistics of spin_lock_mutex from
> > > /proc/lock_stat.
> > > 
> > > Cc: Ingo Molnar <mingo@...nel.org>
> > > Signed-off-by: Yuanhan Liu <yuanhan.liu@...ux.intel.com>
> > > ---
> > >  kernel/mutex-debug.h |    4 ++--
> > >  1 files changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/mutex-debug.h b/kernel/mutex-debug.h
> > > index 0799fd3..556c0bc 100644
> > > --- a/kernel/mutex-debug.h
> > > +++ b/kernel/mutex-debug.h
> > > @@ -43,13 +43,13 @@ static inline void mutex_clear_owner(struct mutex *lock)
> > >  							\
> > >  		DEBUG_LOCKS_WARN_ON(in_interrupt());	\
> > >  		local_irq_save(flags);			\
> > > -		arch_spin_lock(&(lock)->rlock.raw_lock);\
> > > +		spin_lock(lock);			\
> > 
> > But in that case it could probably use the spin_lock_irqsave() 
> > primitive, right?
> 
> Right, in that case I should use spin_lock_irqsave.
> 
> But one question, why we use spin_lock at kernel/mutex.h, 
> while use 'local_irq_save(); arch_spin_lock' at 
> kernel/mutex-debug.h?
> 
> Shouldn't we keep it consistent? Say use spin_lock_irqsave?

I think we did it to increase performance with lockdep enabled - 
this particular lockdep annotation, given the short codepaths, 
is not that hard to verify - and if it breaks it will break a 
thousand mutex locking places in the kernel.

So maybe it's better to leave it alone - maybe add a comment 
that explains the reason.

Thanks,

	Ingo
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