linux-kernel - Re: [tip:locking/core] rwsem: Support optimistic spinning

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Date:	Mon, 19 May 2014 15:39:39 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	mingo@...nel.org
Cc:	torvalds@...ux-foundation.org, peterz@...radead.org,
	peter@...leysoftware.com, jason.low2@...com, alex.shi@...aro.org,
	riel@...hat.com, akpm@...ux-foundation.org, aarcange@...hat.com,
	tglx@...utronix.de, scott.norton@...com,
	linux-kernel@...r.kernel.org, hpa@...or.com, andi@...stfloor.org,
	tim.c.chen@...ux.intel.com, paulmck@...ux.vnet.ibm.com,
	walken@...gle.com, aswin@...com, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:locking/core] rwsem: Support optimistic spinning

On Mon, 2014-05-19 at 14:47 -0700, Davidlohr Bueso wrote:
> Andrew had put this patch in -next for a while, and Stephen Rothwell was
> able to trigger some warnings: https://lkml.org/lkml/2014/5/19/627
> 
> 8<----------------------------------------------------------------
> From: Davidlohr Bueso <davidlohr@...com>
> Subject: [PATCH] rwsem: Fix warnings for CONFIG_RWSEM_GENERIC_SPINLOCK
> 
> Optimistic spinning is only used by the xadd variant
> of rw-semaphores. Make sure that we use the old version
> of the __RWSEM_INITIALIZER macro for systems that rely
> on the spinlock one, otherwise warnings can be triggered,
> such as the following reported on an arm box:
> 
> ipc/ipcns_notifier.c:22:8: warning: excess elements in struct initializer [enabled by default]
> ipc/ipcns_notifier.c:22:8: warning: (near initialization for 'ipcns_chain.rwsem') [enabled by default]
> ipc/ipcns_notifier.c:22:8: warning: excess elements in struct initializer [enabled by default]
> ipc/ipcns_notifier.c:22:8: warning: (near initialization for 'ipcns_chain.rwsem') [enabled by default]
> 
> Signed-off-by: Davidlohr Bueso <davidlohr@...com>
> ---
>  include/linux/rwsem.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 3e108f1..091d993 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -64,7 +64,7 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
>  # define __RWSEM_DEP_MAP_INIT(lockname)
>  #endif
>  
> -#ifdef CONFIG_SMP
> +#if defined(CONFIG_SMP) && defined(CONFIG_RWSEM_XCHGADD_ALGORITHM)

Of course, we can also use the following. I have no preference.

#if defined(CONFIG_SMP) && !defined(CONFIG_RWSEM_GENERIC_SPINLOCK)

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