From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qw0-f51.google.com (mail-qw0-f51.google.com [209.85.216.51]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (verified OK)) by ozlabs.org (Postfix) with ESMTPS id 6E586B6F54 for ; Fri, 15 Jul 2011 20:04:59 +1000 (EST) Received: by qwf7 with SMTP id 7so564245qwf.38 for ; Fri, 15 Jul 2011 03:04:56 -0700 (PDT) Message-ID: <4E20112C.6040307@gmail.com> Date: Fri, 15 Jul 2011 18:06:36 +0800 From: Shan Hai MIME-Version: 1.0 To: Peter Zijlstra Subject: Re: [PATCH 0/1] Fixup write permission of TLB on powerpc e500 core References: <1310717238-13857-1-git-send-email-haishan.bai@gmail.com> <1310718056.2586.275.camel@twins> <4E1FFC7B.4000209@gmail.com> <1310719445.2586.288.camel@twins> <4E20037C.5070506@gmail.com> <1310723441.2586.291.camel@twins> In-Reply-To: <1310723441.2586.291.camel@twins> Content-Type: text/plain; charset=UTF-8; format=flowed Cc: tony.luck@intel.com, 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, akpm@linux-foundation.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 07/15/2011 05:50 PM, Peter Zijlstra wrote: > On Fri, 2011-07-15 at 17:08 +0800, Shan Hai wrote: >> The whole scenario should be, >> - the child process triggers a page fault at the first time access to >> the lock, and it got its own writable page, but its *clean* for >> the reason just for checking the status of the lock. >> I am sorry for above "unbreakable COW". >> - the futex_lock_pi() is invoked because of the lock contention, >> and the futex_atomic_cmpxchg_inatomic() tries to get the lock, >> it found out the lock is free so tries to write to the lock for >> reservation, a page fault occurs, because the page is read only >> for kernel(e500 specific), and returns -EFAULT to the caller >> - the fault_in_user_writeable() tries to fix the fault, >> but from the get_user_pages() view everything is ok, because >> the COW was already broken, retry futex_lock_pi_atomic() > but that's a bug right there, gup(.write=1) _should_ be a complete write > fault, and as such toggle your sw dirty/young tracking. > The fault causing futex_atomic_cmpxchg_inatomic() is protected by pagefault_disable(), so the page fault handler has no chance to toggle the SW dirty/young tracking. Thanks Shan Hai >> - futex_lock_pi_atomic() --> futex_atomic_cmpxchg_inatomic(), >> another write protection page fault >> - infinite loop