linux-kernel - Re: Fw: [GIT PULL] locking fixes

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Message-ID: <1471556063.29842.89.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Thu, 18 Aug 2016 14:34:23 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org
Subject: Re: Fw: [GIT PULL] locking fixes

On Thu, 2016-08-18 at 14:02 -0700, Paul E. McKenney wrote:
> Hello, Eric,
> 
> Does Johannes's recent change to lockless_dereference() help with
> the sparse issue?
> 
> 							Thanx, Paul
> 

Hi Paul

Yes indeed, this helps.

Thanks !

> ----- Forwarded message from Ingo Molnar <mingo@...nel.org> -----
> 
> Date: Thu, 18 Aug 2016 22:34:21 +0200
> From: Ingo Molnar <mingo@...nel.org>
> To: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
> 	Thomas Gleixner <tglx@...utronix.de>, Andrew Morton
> 	<akpm@...ux-foundation.org>, "Paul E. McKenney" <paulmck@...ibm.com>
> Subject: [GIT PULL] locking fixes
> 
> Linus,
> 
> Please pull the latest locking-urgent-for-linus git tree from:
> 
>    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking-urgent-for-linus
> 
>    # HEAD: 112dc0c8069e5554e0ad29c58228f1e6ca49e13d locking/barriers: Suppress sparse warnings in lockless_dereference()
> 
> Two lockless_dereference() related fixes.
> 
>  Thanks,
> 
> 	Ingo
> 
> ------------------>
> Johannes Berg (2):
>       Revert "drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference"
>       locking/barriers: Suppress sparse warnings in lockless_dereference()
> 
> 
>  drivers/gpu/drm/drm_fb_helper.c | 2 +-
>  include/linux/compiler.h        | 6 +++---
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index ce54e985d91b..0a06f9120b5a 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -464,7 +464,7 @@ static bool drm_fb_helper_is_bound(struct drm_fb_helper *fb_helper)
> 
>  	/* Sometimes user space wants everything disabled, so don't steal the
>  	 * display if there's a master. */
> -	if (lockless_dereference(dev->master))
> +	if (READ_ONCE(dev->master))
>  		return false;
> 
>  	drm_for_each_crtc(crtc, dev) {
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 1bb954842725..436aa4e42221 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -527,13 +527,13 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>   * object's lifetime is managed by something other than RCU.  That
>   * "something other" might be reference counting or simple immortality.
>   *
> - * The seemingly unused void * variable is to validate @p is indeed a pointer
> - * type. All pointer types silently cast to void *.
> + * The seemingly unused size_t variable is to validate @p is indeed a pointer
> + * type by making sure it can be dereferenced.
>   */
>  #define lockless_dereference(p) \
>  ({ \
>  	typeof(p) _________p1 = READ_ONCE(p); \
> -	__maybe_unused const void * const _________p2 = _________p1; \
> +	size_t __maybe_unused __size_of_ptr = sizeof(*(p)); \
>  	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
>  	(_________p1); \
>  })
> 
> 
> ----- End forwarded message -----
>