From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759574Ab0EDMlz (ORCPT ); Tue, 4 May 2010 08:41:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39849 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759492Ab0EDMln (ORCPT ); Tue, 4 May 2010 08:41:43 -0400 Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <20100503225105.GA18897@google.com> References: <20100503225105.GA18897@google.com> To: Michel Lespinasse Cc: dhowells@redhat.com, Andrew Morton , LKML Subject: Re: x86 rwsem: up_read() does not check active count in fast path Date: Tue, 04 May 2010 13:41:36 +0100 Message-ID: <30960.1272976896@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Michel Lespinasse wrote: > Looking at the x86 rwsem code, I have been wondering about the up_read() > path. __rwsem_do_wake() comment mentions that one should check the active > count on the way there; however I could not find that check when coming from > up_read(). Look in call_rwsem_wake(), which is implemented in assembly in two places in the x86 arch: arch/x86/lib/rwsem_64.S arch/x86/lib/semaphore_32.S Note that the x86_64 version contains a bug, a patch for which is attached. David --- From: David Howells Subject: [PATCH] Fix the x86_64 implementation of call_rwsem_wait() The x86_64 call_rwsem_wait() treats the active state counter part of the R/W semaphore state as being 16-bit when it's actually 32-bit (it's half of the 64-bit state). It should do "decl %edx" not "decw %dx". Signed-off-by: David Howells --- arch/x86/lib/rwsem_64.S | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-) diff --git a/arch/x86/lib/rwsem_64.S b/arch/x86/lib/rwsem_64.S index 15acecf..41fcf00 100644 --- a/arch/x86/lib/rwsem_64.S +++ b/arch/x86/lib/rwsem_64.S @@ -60,7 +60,7 @@ ENTRY(call_rwsem_down_write_failed) ENDPROC(call_rwsem_down_write_failed) ENTRY(call_rwsem_wake) - decw %dx /* do nothing if still outstanding active readers */ + decl %edx /* do nothing if still outstanding active readers */ jnz 1f save_common_regs movq %rax,%rdi