linux-kernel - Re: [PATCH 05/12] x86 rwsem: take advantage of new RWSEM_WAITING

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Date:	Wed, 12 May 2010 13:10:29 +0100
From:	David Howells <dhowells@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	dhowells@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Waychison <mikew@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 05/12] x86 rwsem: take advantage of new RWSEM_WAITING_BIAS semantics

Michel Lespinasse <walken@...gle.com> wrote:

>  static inline void __up_write(struct rw_semaphore *sem)
>  {
> ...
>  		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"
> -		     /* tries to transition
> -			0xffff0001 -> 0x00000000 */
> -		     "  jz       1f\n"
> +		     /* substracts 0xffff0001, returns the old value */
> +		     "  cmp      %4,%1\n\t"
> +		     /* are there waiting threads and no active threads ? */
> +		     "  jne      1f\n"
>  		     "  call call_rwsem_wake\n"

It looks like you're betting on there being fast-path contention.  If the
common case is no fast-path contention, then you'd be better off putting the
comparison out of line in the medium-path and retaining the JZ instruction.

The same goes for __up_read(): you could retain the JNS there and put the
comparison out of line into the medium-path.

Doing this also saves you some code space.

David
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