linux-kernel - Re: [PATCH] mutex: Fix mutex_can_spin_on

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Date:	Fri, 19 Jul 2013 16:58:09 -0400
From:	Waiman Long <waiman.long@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mutex: Fix mutex_can_spin_on_owner

On 07/19/2013 03:41 PM, Thomas Gleixner wrote:
> On Fri, 19 Jul 2013, Waiman Long wrote:
>> On 07/19/2013 02:31 PM, Peter Zijlstra wrote:
>>>    	rcu_read_lock();
>>> -	if (lock->owner)
>>> -		retval = lock->owner->on_cpu;
>>> +	owner = ACCESS_ONCE(lock->owner);
>>> +	if (owner)
>>> +		retval = owner->on_cpu;
>>>    	rcu_read_unlock();
>>>    	/*
>>>    	 * if lock->owner is not set, the mutex owner may have just
>>> acquired
>> I am fine with this change. However, the compiler is smart enough to
>> not do two memory accesses to the same memory location. So this will
>> not change the generated code. Below is the relevant x86 code for
>> that section of code:
> That's true for your particular compiler, but it's not guaranteed at
> all. So it matters even when your compiler generates the same
> code. Others might not. There is a world outside of x8664.
>
> Thanks,
>
> 	tglx

I supposed that only the gcc compiler can be used to build Linux kernel 
as the kernel source uses a lot of features specific to gcc. 
Optimizations like these are done by the front end of the compiler which 
should be universal across all the architecture. So what I want to say 
is that there is nothing specific to x86-64 or any architecture here. 
This is what a good compiler should do.

I am not against the fix as it makes the intention more clear. I am just 
saying that there won't be any performance change because of this.

Regards,
Longman
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