From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Vrabel Subject: Re: [PATCHv2 4/6] x86: provide xadd() Date: Thu, 16 Apr 2015 12:38:40 +0100 Message-ID: <552F9F40.7030403@citrix.com> References: <1428675597-28465-1-git-send-email-david.vrabel@citrix.com> <1428675597-28465-5-git-send-email-david.vrabel@citrix.com> <20150416112556.GI13443@deinos.phlegethon.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1Yii85-0002Op-4i for xen-devel@lists.xenproject.org; Thu, 16 Apr 2015 11:38:45 +0000 In-Reply-To: <20150416112556.GI13443@deinos.phlegethon.org> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Tim Deegan , David Vrabel Cc: xen-devel@lists.xenproject.org, Keir Fraser , Ian Campbell , Jan Beulich , Stefano Stabellini List-Id: xen-devel@lists.xenproject.org On 16/04/15 12:25, Tim Deegan wrote: > At 15:19 +0100 on 10 Apr (1428679195), David Vrabel wrote: >> xadd() atomically adds a value and returns the previous value. This >> is needed to implement ticket locks. >> >> Signed-off-by: David Vrabel >> --- >> xen/include/asm-x86/system.h | 29 +++++++++++++++++++++++++++++ >> 1 file changed, 29 insertions(+) >> >> diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h >> index 7111329..1e6c6a8 100644 >> --- a/xen/include/asm-x86/system.h >> +++ b/xen/include/asm-x86/system.h >> @@ -117,6 +117,35 @@ static always_inline unsigned long __cmpxchg( >> (unsigned long)__n,sizeof(*(ptr)))); \ >> }) >> >> +static always_inline unsigned long __xadd( >> + volatile void *ptr, unsigned long v, int size) >> +{ >> + switch ( size ) >> + { >> + case 1: >> + asm volatile ( "lock; xaddb %b0,%1" >> + : "+r" (v), "+m" (*__xg((volatile void *)ptr))); >> + return v; >> + case 2: >> + asm volatile ( "lock; xaddw %w0,%1" >> + : "+r" (v), "+m" (*__xg((volatile void *)ptr))); >> + return v; >> + case 4: >> + asm volatile ( "lock; xaddl %k0,%1" >> + : "+r" (v), "+m" (*__xg((volatile void *)ptr))); >> + return v; >> + case 8: >> + asm volatile ( "lock; xaddq %q0,%1" >> + : "+r" (v), "+m" (*__xg((volatile void *)ptr))); >> + return v; >> + } >> + return 0; > > ASSERT_UNREACHABLE(), rather? And some appropriate BUILD_BUG_ON? > > But also: AFAICS the GCC builtin __sync_fetch_and_add() does almost > exactly this (the difference being that those are also compiler > barriers where this is only a CPU barrier). Should we be using it > instead? I wasn't aware of the GCC built-ins -- I'll try them out. I think asm volatile is a compiler barrier though. David