linux-kernel - Re: [patch 1/3] Create spin lock/spin unlock with distinct memory barrier

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Message-ID: <20100201072506.GF9085@laptop>
Date:	Mon, 1 Feb 2010 18:25:06 +1100
From:	Nick Piggin <npiggin@...e.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nicholas Miell <nmiell@...cast.net>, laijs@...fujitsu.com,
	dipankar@...ibm.com, josh@...htriplett.org, dvhltc@...ibm.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [patch 1/3] Create spin lock/spin unlock with distinct memory
 barrier

On Sun, Jan 31, 2010 at 03:52:55PM -0500, Mathieu Desnoyers wrote:
> Create the primitive family:
> 
> spin_lock__no_acquire
> spin_unlock__no_release
> spin_lock_irq__no_acquire
> spin_unlock_irq__no_release
> 
> raw_spin_lock__no_acquire
> raw_spin_unlock__no_release
> raw_spin_lock_irq__no_acquire
> raw_spin_unlock_irq__no_release
> 
> smp_acquire__after_spin_lock()
> smp_release__before_spin_unlock()
> smp_mb__after_spin_lock()
> smp_mb__before_spin_unlock()

Wow, someone who likes micro optimising things as much as I do.
However, these have the wrong names.

smp_mb__after_x() means that calling that function after calling x()
will give a smp_mb(), right?

With your functions, this is giving a smp_mb() after calling
x__no_acquire().

I would suggest maybe just don't bother with the __no_acquire
__no_release variants of spin locks, and stick with the expected
semantics for the new smb_mb__xxx functions. x86 still gets the
full benefit.

But, I don't know if this is even worthwhile, given where you are
using it.

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