linux-kernel - Re: [PATCH] pi-futex: set PF_EXITING without taking ->pi

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Date:	Sat, 21 Jul 2007 17:05:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Eric Dumazet <dada1@...mosbay.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ulrich Drepper <drepper@...hat.com>,
	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...driva.com>
Subject: Re: [PATCH] pi-futex: set PF_EXITING without taking ->pi_lock


* Oleg Nesterov <oleg@...sign.ru> wrote:

> 	static inline void ccids_read_lock(void)
> 	{
> 		atomic_inc(&ccids_lockct);
> 		spin_unlock_wait(&ccids_lock);
> 	}
> 
> This looks racy, in theory atomic_inc() and spin_unlock_wait() could 
> be re-ordered. However, in this particular case we have an "optimized" 
> smp_mb_after_atomic_inc(), perhaps it is good that the caller can 
> choose the "right" barrier by hand.

_all_ default locking and atomic APIs should be barrier-safe i believe. 
(and that includes atomic_inc() too) Most people dont have barriers on 
their mind when their code. _If_ someone is barrier-conscious then we 
should have barrier-less APIs too for that purpose of squeezing the last 
half cycle out of the code, but it should be a non-default choice. The 
reason: nobody notices an unnecessary barrier, but a missing barrier can 
be nasty.

	Ingo
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