linux-kernel - Re: [PATCH] i386: Fix a couple busy loops in mach_wakecpu.h:wait_for_init

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Date:	Fri, 24 Aug 2007 15:25:50 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	"Kenn Humborg" <kenn@...etree.ie>
Cc:	"Satyam Sharma" <satyam@...radead.org>,
	"Heiko Carstens" <heiko.carstens@...ibm.com>,
	"Herbert Xu" <herbert@...dor.apana.org.au>,
	"Chris Snook" <csnook@...hat.com>, clameter@....com,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	netdev@...r.kernel.org,
	"Andrew Morton" <akpm@...ux-foundation.org>, ak@...e.de,
	davem@...emloft.net, schwidefsky@...ibm.com, wensong@...ux-vs.org,
	horms@...ge.net.au, wjiang@...ilience.com, cfriesen@...tel.com,
	zlynx@....org, rpjday@...dspring.com, jesper.juhl@...il.com,
	segher@...nel.crashing.org
Subject: Re: [PATCH] i386: Fix a couple busy loops in mach_wakecpu.h:wait_for_init_deassert()

On Friday 24 August 2007 13:12, Kenn Humborg wrote:
> > On Thursday 16 August 2007 01:39, Satyam Sharma wrote:
> > >  static inline void wait_for_init_deassert(atomic_t *deassert)
> > >  {
> > > -	while (!atomic_read(deassert));
> > > +	while (!atomic_read(deassert))
> > > +		cpu_relax();
> > >  	return;
> > >  }
> >
> > For less-than-briliant people like me, it's totally non-obvious that
> > cpu_relax() is needed for correctness here, not just to make P4 happy.
> >
> > IOW: "atomic_read" name quite unambiguously means "I will read
> > this variable from main memory". Which is not true and creates
> > potential for confusion and bugs.
>
> To me, "atomic_read" means a read which is synchronized with other
> changes to the variable (using the atomic_XXX functions) in such
> a way that I will always only see the "before" or "after"
> state of the variable - never an intermediate state while a
> modification is happening.  It doesn't imply that I have to
> see the "after" state immediately after another thread modifies
> it.

So you are ok with compiler propagating n1 to n2 here:

n1 += atomic_read(x);
other_variable++;
n2 += atomic_read(x);

without accessing x second time. What's the point? Any sane coder
will say that explicitly anyway:

tmp = atomic_read(x);
n1 += tmp;
other_variable++;
n2 += tmp;

if only for the sake of code readability. Because first code
is definitely hinting that it reads RAM twice, and it's actively *bad*
for code readability when in fact it's not the case!

Locking, compiler and CPU barriers are complicated enough already,
please don't make them even harder to understand.
--
vda
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