linux-kernel - Re: [PATCH -tip/master 7/7] Documentation: Update locking/mutex-design.txt disadvantages

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Date:	Mon, 28 Jul 2014 11:09:27 -0700
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	peterz@...radead.org, mingo@...nel.org, aswin@...com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip/master 7/7] Documentation: Update
 locking/mutex-design.txt disadvantages

On Sun, 2014-07-27 at 22:18 -0700, Davidlohr Bueso wrote:
> Fortunately Jason was able to reduce some of the overhead we
> had introduced in the original rwsem optimistic spinning -
> an it is now the same size as mutexes. Update the documentation
> accordingly.
> 
> Signed-off-by: Davidlohr Bueso <davidlohr@...com>
> ---
>  Documentation/locking/mutex-design.txt | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/locking/mutex-design.txt b/Documentation/locking/mutex-design.txt
> index ee231ed..60c482d 100644
> --- a/Documentation/locking/mutex-design.txt
> +++ b/Documentation/locking/mutex-design.txt
> @@ -145,9 +145,9 @@ Disadvantages
>  
>  Unlike its original design and purpose, 'struct mutex' is larger than
>  most locks in the kernel. E.g: on x86-64 it is 40 bytes, almost twice
> -as large as 'struct semaphore' (24 bytes) and 8 bytes shy of the
> -'struct rw_semaphore' variant. Larger structure sizes mean more CPU
> -cache and memory footprint.
> +as large as 'struct semaphore' (24 bytes) and tied, along with rwsems,

My suggestion here is for the above to say 'struct rw_semaphore' instead
of rwsems.

Acked-by: Jason Low <jason.low2@...com>

> +for the largest lock in the kernel. Larger structure sizes mean more
> +CPU cache and memory footprint.
>  
>  When to use mutexes
>  -------------------


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