linux-kernel - Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long

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Message-ID: <1393478904.3884.6.camel@buesod1.americas.hpqcorp.net>
Date:	Wed, 26 Feb 2014 21:28:24 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-kernel@...r.kernel.org, rkuo@...eaurora.org, arnd@...db.de,
	paulus@...ba.org
Subject: Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only
 accessed via atomic_long_*

On Fri, 2014-02-21 at 17:22 +0000, Will Deacon wrote:
> The asm-generic rwsem implementation directly acceses sem->cnt when
> performing a __down_read_trylock operation. Whilst this is probably safe
> on all architectures, we should stick to the atomic_long_* API and use
> atomic_long_read instead.
> 
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
>  include/asm-generic/rwsem.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> index bb1e2cdeb9bf..75af612f54f8 100644
> --- a/include/asm-generic/rwsem.h
> +++ b/include/asm-generic/rwsem.h
> @@ -41,7 +41,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
>  {
>  	long tmp;
>  
> -	while ((tmp = sem->count) >= 0) {
> +	while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) {

That's pretty ugly, how about having infinite look and just do the tmp
assign separately from the conditional?

It also looks like a cpu_relax() could help here between iterations.
Other than that:

Reviewed-by: Davidlohr Bueso <davidlohr@...com>

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