From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.linux-foundation.org (smtp1.linux-foundation.org [140.211.169.13]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "smtp.linux-foundation.org", Issuer "CA Cert Signing Authority" (verified OK)) by ozlabs.org (Postfix) with ESMTPS id DB1CBB6F77 for ; Fri, 22 Jul 2011 09:00:01 +1000 (EST) Date: Thu, 21 Jul 2011 15:59:38 -0700 From: Andrew Morton To: Benjamin Herrenschmidt Subject: Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW tracking of dirty & young Message-Id: <20110721155938.2ff2dab5.akpm@linux-foundation.org> In-Reply-To: <1311288726.25044.545.camel@pasglop> References: <1310717238-13857-1-git-send-email-haishan.bai@gmail.com> <1310717238-13857-2-git-send-email-haishan.bai@gmail.com> <1310725418.2586.309.camel@twins> <4E21A526.8010904@gmail.com> <1310860194.25044.17.camel@pasglop> <4b337921-d430-4b63-bc36-ad31753cf800@email.android.com> <1310912990.25044.203.camel@pasglop> <1310944453.25044.262.camel@pasglop> <1310961691.25044.274.camel@pasglop> <4E23D728.7090406@gmail.com> <1310972462.25044.292.camel@pasglop> <4E23E02C.8090401@gmail.com> <1310974591.25044.298.camel@pasglop> <4E24FA51.70602@gmail.com> <1311049762.25044.392.camel@pasglop> <20110721153606.37e6f432.akpm@linux-foundation.org> <1311288726.25044.545.camel@pasglop> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Cc: tony.luck@intel.com, Peter Zijlstra , Shan Hai , Peter Zijlstra , linux-kernel@vger.kernel.org, cmetcalf@tilera.com, dhowells@redhat.com, paulus@samba.org, tglx@linutronix.de, walken@google.com, linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Fri, 22 Jul 2011 08:52:06 +1000 Benjamin Herrenschmidt wrote: > On Thu, 2011-07-21 at 15:36 -0700, Andrew Morton wrote: > > On Tue, 19 Jul 2011 14:29:22 +1000 > > Benjamin Herrenschmidt wrote: > > > > > The futex code currently attempts to write to user memory within > > > a pagefault disabled section, and if that fails, tries to fix it > > > up using get_user_pages(). > > > > > > This doesn't work on archs where the dirty and young bits are > > > maintained by software, since they will gate access permission > > > in the TLB, and will not be updated by gup(). > > > > > > In addition, there's an expectation on some archs that a > > > spurious write fault triggers a local TLB flush, and that is > > > missing from the picture as well. > > > > > > I decided that adding those "features" to gup() would be too much > > > for this already too complex function, and instead added a new > > > simpler fixup_user_fault() which is essentially a wrapper around > > > handle_mm_fault() which the futex code can call. > > > > > > Signed-off-by: Benjamin Herrenschmidt > > > --- > > > > > > Shan, can you test this ? It might not fix the problem > > > > um, what problem. There's no description here of the user-visible > > effects of the bug hence it's hard to work out what kernel version(s) > > should receive this patch. > > Shan could give you an actual example (it was in the previous thread), > but basically, livelock as the kernel keeps trying and trying the > in_atomic op and never resolves it. > > > What kernel version(s) should receive this patch? > > I haven't dug. Probably anything it applies on as far as we did that > trick of atomic + gup() for futex. You're not understanding me. I need a good reason to merge this into 3.0. The -stable maintainers need even better reasons to merge this into earlier kernels. Please provide those reasons! (Documentation/stable_kernel_rules.txt, 4th bullet) (And it's not just me and -stable maintainers. Distro maintainers will also look at this patch and wonder whether they should merge it)