linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Date:	Thu, 16 Aug 2007 13:48:32 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Christoph Lameter <clameter@....com>,
	Satyam Sharma <satyam@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Chris Snook <csnook@...hat.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, heiko.carstens@...ibm.com, 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 0/24] make atomic_read() behave consistently across all architectures

Herbert Xu writes:

> If you're referring to the code in sk_stream_mem_schedule
> then it's working as intended.  The atomicity guarantees

You mean it's intended that *sk->sk_prot->memory_pressure can end up
as 1 when sk->sk_prot->memory_allocated is small (less than
->sysctl_mem[0]), or as 0 when ->memory_allocated is large (greater
than ->sysctl_mem[2])?  Because that's the effect of the current code.
If so I wonder why you bother computing it.

> that the atomic_add/atomic_sub won't be seen in parts by
> other readers.
> 
> We certainly do not need to see other atomic_add/atomic_sub
> operations immediately.
> 
> If you're referring to another code snippet please cite.
> 
> > I'd go so far as to say that anywhere where you want a non-"volatile"
> > atomic_read, either your code is buggy, or else an int would work just
> > as well.
> 
> An int won't work here because += and -= do not have the
> atomicity guarantees that atomic_add/atomic_sub do.  In
> particular, this may cause an atomic_read on another CPU
> to give a bogus reading.

The point is that guaranteeing the atomicity of the increment or
decrement does not suffice to make the code race-free.  In this case
the race arises from the fact that reading ->memory_allocated and
setting *->memory_pressure are separate operations.  To make that code
work properly you need a lock.  And once you have the lock an ordinary
int would suffice for ->memory_allocated.

Paul.
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