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

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Date:	Fri, 25 Jan 2013 08:40:53 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/2] mutex: use spin_[un]lock instead of
 arch_spin_[un]lock


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Thu, 24 Jan 2013 17:22:45 +0800
> 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.

So, as per the discussion we don't want this patch, because we 
are using raw locks there to keep mutex lockdep overhead low. 
The value of lockdep-checking such a basic locking primitive is 
minimal - it's rarely tweaked and if it breaks we won't have a 
bootable kernel to begin with.

So instead I suggested a different patch: adding a comment to 
explain why we don't lockdep-cover the mutex code spinlocks.

> Also, I believe your patch permits this cleanup:
> 
> --- a/kernel/mutex-debug.h~mutex-use-spin_lock-instead-of-arch_spin_lock-fix
> +++ a/kernel/mutex-debug.h
> @@ -42,14 +42,12 @@ static inline void mutex_clear_owner(str
>  		struct mutex *l = container_of(lock, struct mutex, wait_lock); \
>  							\
>  		DEBUG_LOCKS_WARN_ON(in_interrupt());	\
> -		local_irq_save(flags);			\
> -		spin_lock(lock);			\
> +		spin_lock_irqsave(lock, flags);		\

Yes, I mentioned that yesterday, but we really don't want the 
change to begin with.

Thanks,

	Ingo
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