From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from sc8-sf-mx1-b.sourceforge.net ([10.3.1.91] helo=mail.sourceforge.net) by sc8-sf-list1-new.sourceforge.net with esmtp (Exim 4.43) id 1GYXAQ-0005RD-Cb for user-mode-linux-devel@lists.sourceforge.net; Fri, 13 Oct 2006 17:14:20 -0700 Received: from smtp001.mail.ukl.yahoo.com ([217.12.11.32]) by mail.sourceforge.net with smtp (Exim 4.44) id 1GYXAM-00020f-GD for user-mode-linux-devel@lists.sourceforge.net; Fri, 13 Oct 2006 17:14:18 -0700 From: Blaisorblade Date: Sat, 14 Oct 2006 02:13:55 +0200 References: <20061005213212.17268.7409.stgit@memento.home.lan> <20061005213852.17268.13871.stgit@memento.home.lan> <20061009180013.GB4931@ccure.user-mode-linux.org> In-Reply-To: <20061009180013.GB4931@ccure.user-mode-linux.org> MIME-Version: 1.0 Content-Disposition: inline Message-Id: <200610140213.56153.blaisorblade@yahoo.it> Subject: Re: [uml-devel] [PATCH 06/14] uml: make UML_SETJMP always safe List-Id: The user-mode Linux development list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: user-mode-linux-devel-bounces@lists.sourceforge.net Errors-To: user-mode-linux-devel-bounces@lists.sourceforge.net To: Jeff Dike , Alexander Viro Cc: Andrew Morton , linux-kernel@vger.kernel.org, user-mode-linux-devel@lists.sourceforge.net On Monday 09 October 2006 20:00, Jeff Dike wrote: > On Thu, Oct 05, 2006 at 11:38:52PM +0200, Paolo 'Blaisorblade' Giarrusso wrote: > > From: Paolo 'Blaisorblade' Giarrusso > > > > If enable is moved by GCC in a register its value may not be preserved > > after coming back there with longjmp(). So, mark it as volatile to > > prevent this; this is suggested (it seems) in info gcc, when it talks > > about -Wuninitialized. I re-read this and it seems to say something > > different, but I still believe this may be needed. > > > > Signed-off-by: Paolo 'Blaisorblade' Giarrusso > > --- > > > > arch/um/include/longjmp.h | 3 ++- > > 1 files changed, 2 insertions(+), 1 deletions(-) > > > > diff --git a/arch/um/include/longjmp.h b/arch/um/include/longjmp.h > > index e93c6d3..e860bc5 100644 > > --- a/arch/um/include/longjmp.h > > +++ b/arch/um/include/longjmp.h > > @@ -12,7 +12,8 @@ #define UML_LONGJMP(buf, val) do { \ > > } while(0) > > > > #define UML_SETJMP(buf) ({ \ > > - int n, enable; \ > > + int n; \ > > + volatile int enable; \ > > enable = get_signals(); \ > > n = setjmp(*buf); \ > > if(n != 0) \ > > I agree with this, but not entirely with your reasoning. The > -Wuninitialized documentation just talks about when gcc emits a > warning. > > What we want is a guarantee that enable is not cached in a register, > but is stored in memory. What documentation I can find seems to imply > that is the case ("accesses to volatile objects must have settled > before the next sequence point"). > > However, given the prevailing opinion that essentially all volatile > declarations are hiding bugs, I wouldn't mind a bit of review of this > from someone holding this opinion. I agree with that, so I think Al (whom I cc'ed) will be able to evaluate this. - however I think that the problem with volatile is that it does not have any semantic wrt cache - volatile does not even work with host-to-device operation (the thing it was thought for) if you have reasonable caches - unless the var is in a MMIO region with disabled caches. So relying on volatile for variables shared between threads is not useful. Note that raw_spinlock_t counter is volatile, however - separate caches flushes or atomic operations are needed, but it may avoid the need for a full barrier() operation or a volatile "memory" clobber - GCC knows only the volatile object has changed, while with barrier() it must assume everything must be reloaded. However, at least one barrier operation is needed for a spinlock to prevent GCC from moving code outside of critical sections. -- Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!". Paolo Giarrusso, aka Blaisorblade http://www.user-mode-linux.org/~blaisorblade Chiacchiera con i tuoi amici in tempo reale! http://it.yahoo.com/mail_it/foot/*http://it.messenger.yahoo.com ------------------------------------------------------------------------- Using Tomcat but need to do more? Need to support web services, security? Get stuff done quickly with pre-integrated technology to make your job easier Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642 _______________________________________________ User-mode-linux-devel mailing list User-mode-linux-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/user-mode-linux-devel